From d2b8a32ade53f062b259142125dd6da02f069333 Mon Sep 17 00:00:00 2001
From: Luca Guidi <me@lucaguidi.com>
Date: Wed, 29 Nov 2017 12:15:12 +0100
Subject: [PATCH] Remove global state. Immutable mailer. (#69)

---
 .rubocop.yml                                  |   4 +
 Gemfile                                       |   5 +-
 README.md                                     | 155 +++---
 benchmark.rb                                  |  48 ++
 config/flay.yml                               |   3 -
 config/flog.yml                               |   2 +-
 config/reek.yml                               |  43 +-
 config/rubocop.yml                            |  97 ----
 examples/base.rb                              |  46 ++
 examples/base/invoice.html.erb                |   8 +
 examples/base/invoice.txt.erb                 |   1 +
 hanami-mailer.gemspec                         |   2 +-
 lib/hanami-mailer.rb                          |   1 -
 lib/hanami/mailer.rb                          | 436 +++++++----------
 lib/hanami/mailer/configuration.rb            | 265 +++++------
 lib/hanami/mailer/dsl.rb                      | 441 +++++++-----------
 lib/hanami/mailer/finalizer.rb                |  32 ++
 lib/hanami/mailer/rendering/template_name.rb  |  53 ---
 .../mailer/rendering/templates_finder.rb      | 133 ------
 lib/hanami/mailer/template.rb                 |  21 +-
 lib/hanami/mailer/template_name.rb            |  25 +
 lib/hanami/mailer/templates_finder.rb         | 152 ++++++
 lib/hanami/mailer/version.rb                  |   6 +-
 .../hanami/mailer/delivery_spec.rb            | 154 ++++++
 spec/spec_helper.rb                           |  30 +-
 spec/support/context.rb                       |  22 +
 spec/support/fixtures.rb                      |  94 ++--
 ...hod_mailer.txt.erb => proc_mailer.txt.erb} |   0
 .../support/fixtures/templates/welcome_mailer |   2 +
 .../fixtures/templates/welcome_mailer.erb     |   2 +
 .../fixtures/templates/welcome_mailer.html    |   2 +
 spec/unit/hanami/mailer/configuration_spec.rb | 145 ++----
 spec/unit/hanami/mailer/delivery_spec.rb      | 184 --------
 spec/unit/hanami/mailer/dsl_spec.rb           | 146 ++++--
 spec/unit/hanami/mailer/error_spec.rb         |   7 +
 spec/unit/hanami/mailer/finalizer_spec.rb     |  28 ++
 .../missing_delivery_data_error_spec.rb       |  11 +
 spec/unit/hanami/mailer/rendering_spec.rb     |  44 --
 spec/unit/hanami/mailer/template_name_spec.rb |  67 +++
 spec/unit/hanami/mailer/template_spec.rb      |  46 ++
 .../hanami/mailer/templates_finder_spec.rb    |  75 +++
 .../mailer/unknown_mailer_error_spec.rb       |  17 +
 spec/unit/hanami/mailer/version_spec.rb       |   2 +
 spec/unit/hanami/mailer_spec.rb               | 168 +++++++
 44 files changed, 1725 insertions(+), 1500 deletions(-)
 create mode 100644 benchmark.rb
 delete mode 100644 config/flay.yml
 delete mode 100644 config/rubocop.yml
 create mode 100644 examples/base.rb
 create mode 100644 examples/base/invoice.html.erb
 create mode 100644 examples/base/invoice.txt.erb
 delete mode 100644 lib/hanami-mailer.rb
 create mode 100644 lib/hanami/mailer/finalizer.rb
 delete mode 100644 lib/hanami/mailer/rendering/template_name.rb
 delete mode 100644 lib/hanami/mailer/rendering/templates_finder.rb
 create mode 100644 lib/hanami/mailer/template_name.rb
 create mode 100644 lib/hanami/mailer/templates_finder.rb
 create mode 100644 spec/integration/hanami/mailer/delivery_spec.rb
 create mode 100644 spec/support/context.rb
 rename spec/support/fixtures/templates/{method_mailer.txt.erb => proc_mailer.txt.erb} (100%)
 create mode 100644 spec/support/fixtures/templates/welcome_mailer
 create mode 100644 spec/support/fixtures/templates/welcome_mailer.erb
 create mode 100644 spec/support/fixtures/templates/welcome_mailer.html
 delete mode 100644 spec/unit/hanami/mailer/delivery_spec.rb
 create mode 100644 spec/unit/hanami/mailer/error_spec.rb
 create mode 100644 spec/unit/hanami/mailer/finalizer_spec.rb
 create mode 100644 spec/unit/hanami/mailer/missing_delivery_data_error_spec.rb
 delete mode 100644 spec/unit/hanami/mailer/rendering_spec.rb
 create mode 100644 spec/unit/hanami/mailer/template_name_spec.rb
 create mode 100644 spec/unit/hanami/mailer/template_spec.rb
 create mode 100644 spec/unit/hanami/mailer/templates_finder_spec.rb
 create mode 100644 spec/unit/hanami/mailer/unknown_mailer_error_spec.rb
 create mode 100644 spec/unit/hanami/mailer_spec.rb

diff --git a/.rubocop.yml b/.rubocop.yml
index 9790ad7..474ebbb 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -2,3 +2,7 @@
 # alphabetically
 inherit_from:
   - https://raw.githubusercontent.com/hanami/devtools/master/.rubocop.yml
+Style/Documentation:
+  Exclude:
+    - "examples/*"
+    - "spec/**/*"
diff --git a/Gemfile b/Gemfile
index ce9d366..38326fe 100644
--- a/Gemfile
+++ b/Gemfile
@@ -2,8 +2,9 @@ source 'https://rubygems.org'
 gemspec
 
 unless ENV['TRAVIS']
-  gem 'byebug', require: false, platforms: :mri
-  gem 'yard',   require: false
+  gem 'byebug',           require: false, platforms: :mri
+  gem 'allocation_stats', require: false
+  gem 'benchmark-ips',    require: false
 end
 
 gem 'hanami-utils', '2.0.0.alpha1', require: false, git: 'https://github.com/hanami/utils.git', branch: 'unstable'
diff --git a/README.md b/README.md
index 1bbdafd..f5dabe2 100644
--- a/README.md
+++ b/README.md
@@ -44,10 +44,10 @@ Or install it yourself as:
 
 ### Conventions
 
-  * Templates are searched under `Hanami::Mailer.configuration.root`, set this value according to your app structure (eg. `"app/templates"`).
+  * Templates are searched under `Hanami::Mailer::Configuration#root`, set this value according to your app structure (eg. `"app/templates"`).
   * A mailer will look for a template with a file name that is composed by its full class name (eg. `"articles/index"`).
   * A template must have two concatenated extensions: one for the format and one for the engine (eg. `".html.erb"`).
-  * The framework must be loaded before rendering the first time: `Hanami::Mailer.load!`.
+  * The framework must be loaded before rendering the first time: `Hanami::Mailer.finalize(configuration)`.
 
 ### Mailers
 
@@ -55,10 +55,60 @@ A simple mailer looks like this:
 
 ```ruby
 require 'hanami/mailer'
+require 'ostruct'
 
-class InvoiceMailer
-  include Hanami::Mailer
+# Create two files: `invoice.html.erb` and `invoice.txt.erb`
+
+configuration = Hanami::Mailer::Configuration.new do |config|
+  config.delivery_method = :test
 end
+
+class Invoice < Hanami::Mailer
+  from "noreply@example.com"
+  to ->(locals) { locals.fetch(:user).email }
+end
+
+configuration = Hanami::Mailer.finalize(configuration)
+
+invoice = OpenStruct.new(number: 23)
+mailer  = InvoiceMailer.new(configuration: configuration)
+mail    = mailer.deliver(invoice: invoice)
+
+mail
+  # => #<Mail::Message:70303354246540, Multipart: true, Headers: <Date: Wed, 22 Mar 2017 11:48:57 +0100>, <From: noreply@example.com>, <To: user@example.com>, <Cc: >, <Bcc: >, <Message-ID: <58d25699e47f9_b4e13ff0c503e4f4632e6@escher.mail>>, <Subject: >, <Mime-Version: 1.0>, <Content-Type: multipart/alternative; boundary=--==_mimepart_58d25699e42d2_b4e13ff0c503e4f463186>, <Content-Transfer-Encoding: 7bit>>
+
+mail.to_s
+  # =>
+  # From: noreply@example.com
+  # To: user@example.com
+  # Message-ID: <58d25699e47f9_b4e13ff0c503e4f4632e6@escher.mail>
+  # Subject:
+  # Mime-Version: 1.0
+  # Content-Type: multipart/alternative;
+  #  boundary="--==_mimepart_58d25699e42d2_b4e13ff0c503e4f463186";
+  #  charset=UTF-8
+  # Content-Transfer-Encoding: 7bit
+  #
+  #
+  # ----==_mimepart_58d25699e42d2_b4e13ff0c503e4f463186
+  # Content-Type: text/plain;
+  #  charset=UTF-8
+  # Content-Transfer-Encoding: 7bit
+  #
+  # Invoice #23
+  #
+  # ----==_mimepart_58d25699e42d2_b4e13ff0c503e4f463186
+  # Content-Type: text/html;
+  #  charset=UTF-8
+  # Content-Transfer-Encoding: 7bit
+  #
+  # <html>
+  #   <body>
+  #     <h1>Invoice template</h1>
+  #   </body>
+  # </html>
+  #
+  # ----==_mimepart_58d25699e42d2_b4e13ff0c503e4f463186--
 ```
 
 A mailer with `.to` and `.from` addresses and mailer delivery:
@@ -66,20 +116,18 @@ A mailer with `.to` and `.from` addresses and mailer delivery:
 ```ruby
 require 'hanami/mailer'
 
-Hanami::Mailer.configure do
-  delivery_method :smtp,
-    address:              "smtp.gmail.com",
-    port:                 587,
-    domain:               "example.com",
-    user_name:            ENV['SMTP_USERNAME'],
-    password:             ENV['SMTP_PASSWORD'],
-    authentication:       "plain",
-    enable_starttls_auto: true
-end.load!
-
-class WelcomeMailer
-  include Hanami::Mailer
+configuration = Hanami::Mailer::Configuration.new do |config|
+  config.delivery_method = :smtp,
+                           address:              "smtp.gmail.com",
+                           port:                 587,
+                           domain:               "example.com",
+                           user_name:            ENV['SMTP_USERNAME'],
+                           password:             ENV['SMTP_PASSWORD'],
+                           authentication:       "plain",
+                           enable_starttls_auto: true
+end
 
+class WelcomeMailer < Hanami::Mailer
   from 'noreply@sender.com'
   to   'noreply@recipient.com'
   cc   'cc@sender.com'
@@ -88,7 +136,7 @@ class WelcomeMailer
   subject 'Welcome'
 end
 
-WelcomeMailer.deliver
+WelcomeMailer.new(configuration: configuration).call(locals)
 ```
 
 ### Locals
@@ -97,25 +145,17 @@ The set of objects passed in the `deliver` call are called `locals` and are avai
 
 ```ruby
 require 'hanami/mailer'
+require 'ostruct'
 
-User = Struct.new(:name, :username, :email)
-luca = User.new('Luca', 'jodosha', 'luca@jodosha.com')
-
-class WelcomeMailer
-  include Hanami::Mailer
+user = OpenStruct.new(name: Luca', email: 'user@hanamirb.org')
 
+class WelcomeMailer < Hanami::Mailer
   from    'noreply@sender.com'
   subject 'Welcome'
-  to      :recipient
-
-  private
-
-  def recipient
-    user.email
-  end
+  to      ->(locals) { locals.fetch(:user).email }
 end
 
-WelcomeMailer.deliver(user: luca)
+WelcomeMailer.new(configuration: configuration).deliver(user: luca)
 ```
 
 The corresponding `erb` file:
@@ -131,9 +171,7 @@ All public methods defined in the mailer are accessible from the template:
 ```ruby
 require 'hanami/mailer'
 
-class WelcomeMailer
-  include Hanami::Mailer
-
+class WelcomeMailer < Hanami::Mailer
   from    'noreply@sender.com'
   to      'noreply@recipient.com'
   subject 'Welcome'
@@ -154,7 +192,7 @@ The template file must be located under the relevant `root` and must match the i
 
 ```ruby
 # Given this root
-Hanami::Mailer.configuration.root      # => #<Pathname:app/templates>
+configuration.root      # => #<Pathname:app/templates>
 
 # For InvoiceMailer, it looks for:
 #  * app/templates/invoice_mailer.html.erb
@@ -164,9 +202,7 @@ Hanami::Mailer.configuration.root      # => #<Pathname:app/templates>
 If we want to specify a different template, we can do:
 
 ```ruby
-class InvoiceMailer
-  include Hanami::Mailer
-
+class InvoiceMailer < Hanami::Mailer
   template 'invoice'
 end
 
@@ -311,21 +347,22 @@ It supports a few options:
 ```ruby
 require 'hanami/mailer'
 
-Hanami::Maler.configure do
+configuration = Hanami::Mailer::Configuration.new do |config|
   # Set the root path where to search for templates
   # Argument: String, Pathname, #to_pathname, defaults to the current directory
   #
-  root '/path/to/root'
+  config.root = 'path/to/root'
 
   # Set the default charset for emails
   # Argument: String, defaults to "UTF-8"
   #
-  default_charset 'iso-8859'
+  config.default_charset = 'iso-8859'
 
   # Set the delivery method
   # Argument: Symbol
   # Argument: Hash, optional configurations
-  delivery_method :stmp
+  config.delivery_method = :stmp
+end
 ```
 
 ### Attachments
@@ -333,14 +370,10 @@ Hanami::Maler.configure do
 Attachments can be added with the following API:
 
 ```ruby
-class InvoiceMailer
-  include Hanami::Mailer
+class InvoiceMailer < Hanami::Mailer
   # ...
-
-  def prepare
-    mail.attachments['invoice.pdf'] = '/path/to/invoice.pdf'
-    # or
-    mail.attachments['invoice.pdf'] = File.read('/path/to/invoice.pdf')
+  before do |mail, locals|
+    mail.attachments["invoice-#{locals.fetch(:invoice).number}.pdf"] = 'path/to/invoice.pdf'
   end
 end
 ```
@@ -350,14 +383,14 @@ end
 The global delivery method is defined through the __Hanami::Mailer__ configuration, as:
 
 ```ruby
-Hanami::Mailer.configuration do
-  delivery_method :smtp
+configuration = Hanami::Mailer::Configuration.new do |config|
+  config.delivery_method = :smtp
 end
 ```
 
 ```ruby
-Hanami::Mailer.configuration do
-  delivery_method :smtp, address: "localhost", port: 1025
+configuration = Hanami::Mailer::Configuration.new do |config|
+  config.delivery_method = :smtp, { address: "localhost", port: 1025 }
 end
 ```
 
@@ -386,14 +419,14 @@ class MandrillDeliveryMethod
   end
 end
 
-Hanami::Mailer.configure do
-  delivery_method MandrillDeliveryMethod,
-    username: ENV['MANDRILL_USERNAME'],
-    password: ENV['MANDRILL_API_KEY']
-end.load!
+configuration = Hanami::Mailer::Configuration.new do |config|
+  config.delivery_method = MandrillDeliveryMethod,
+                           username: ENV['MANDRILL_USERNAME'],
+                           password: ENV['MANDRILL_API_KEY']
+end
 ```
 
-The class passed to `.delivery_method` must accept an optional set of options
+The class passed to `.delivery_method=` must accept an optional set of options
 with the constructor (`#initialize`) and respond to `#deliver!`.
 
 ### Multipart Delivery
@@ -402,8 +435,8 @@ All the email are sent as multipart messages by default.
 For a given mailer, the framework looks up for associated text (`.txt`) and `HTML` (`.html`) templates and render them.
 
 ```ruby
-InvoiceMailer.deliver               # delivers both text and html templates
-InvoiceMailer.deliver(format: :txt) # delivers only text template
+InvoiceMailer.new(configuration: configuration).deliver({})           # delivers both text and html templates
+InvoiceMailer.new(configuration: configuration).deliver(format: :txt) # delivers only text template
 ```
 
 Please note that **they aren't both mandatory, but at least one of them MUST** be present.
diff --git a/benchmark.rb b/benchmark.rb
new file mode 100644
index 0000000..d73445b
--- /dev/null
+++ b/benchmark.rb
@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+require 'bundler/setup'
+require 'hanami/mailer'
+require 'benchmark/ips'
+require 'allocation_stats'
+require_relative './examples/base'
+
+configuration = Hanami::Mailer::Configuration.new do |config|
+  config.root            = "examples/base"
+  config.delivery_method = :test
+end
+
+configuration = Hanami::Mailer.finalize(configuration)
+
+invoice = Invoice.new(1, 23)
+user    = User.new("Luca", "luca@domain.test")
+
+mailer = InvoiceMailer.new(configuration: configuration)
+
+Benchmark.ips do |x|
+  # # Configure the number of seconds used during
+  # # the warmup phase (default 2) and calculation phase (default 5)
+  # x.config(time: 5, warmup: 2)
+  x.report "deliver" do
+    mailer.deliver(invoice: invoice, user: user)
+  end
+end
+
+stats = AllocationStats.new(burn: 5).trace do
+  1_000.times do
+    mailer.deliver(invoice: invoice, user: user)
+  end
+end
+
+total_allocations = stats.allocations.all.size
+puts "total allocations: #{total_allocations}"
+
+total_memsize = stats.allocations.bytes.to_a.inject(&:+)
+puts "total memsize: #{total_memsize}"
+
+detailed_allocations = stats.allocations(alias_paths: true)
+                            .group_by(:sourcefile, :class_plus)
+                            .sort_by_count
+                            .to_text
+
+puts 'allocations by source file and class:'
+puts detailed_allocations
diff --git a/config/flay.yml b/config/flay.yml
deleted file mode 100644
index d55b4e3..0000000
--- a/config/flay.yml
+++ /dev/null
@@ -1,3 +0,0 @@
----
-threshold: 10
-total_score: 77
diff --git a/config/flog.yml b/config/flog.yml
index e97b949..db27c00 100644
--- a/config/flog.yml
+++ b/config/flog.yml
@@ -1,2 +1,2 @@
 ---
-threshold: 20.2
+threshold: 18.4
diff --git a/config/reek.yml b/config/reek.yml
index 28474d4..e673d62 100644
--- a/config/reek.yml
+++ b/config/reek.yml
@@ -25,12 +25,12 @@ DuplicateMethodCall:
 FeatureEnvy:
   enabled: true
   exclude:
-  - Hanami::Mailer#build
+  - Hanami::Mailer#__part?
 LongParameterList:
   enabled: true
   exclude:
   - Devtools::Config#self.attribute
-  max_params: 2
+  max_params: 4
   overrides: {}
 LongYieldList:
   enabled: true
@@ -44,28 +44,23 @@ NestedIterators:
 NilCheck:
   enabled: true
   exclude:
-  - Hanami::Mailer::Configuration#default_charset
-  - Hanami::Mailer::Configuration#delivery_method
   - Hanami::Mailer::Dsl#bcc
   - Hanami::Mailer::Dsl#cc
   - Hanami::Mailer::Dsl#from
   - Hanami::Mailer::Dsl#subject
-  - Hanami::Mailer::Dsl#template
-  - Hanami::Mailer::Dsl#templates
   - Hanami::Mailer::Dsl#to
   - Hanami::Mailer#__part?
 RepeatedConditional:
   enabled: true
   max_ifs: 1
-  exclude:
-    - Hanami::Mailer::Configuration
+  exclude: []
 TooManyConstants:
   enabled: true
   exclude:
   - Devtools
 TooManyInstanceVariables:
   enabled: true
-  max_instance_variables: 2
+  max_instance_variables: 3
   exclude:
   - Hanami::Mailer::Configuration
 TooManyMethods:
@@ -76,12 +71,10 @@ TooManyStatements:
   enabled: true
   max_statements: 5
   exclude:
-  - initialize
-  - Hanami::Mailer::Configuration#duplicate
-  - Hanami::Mailer::Dsl#self.extended
-  - Hanami::Mailer::Rendering::TemplatesFinder#find
-  - Hanami::Mailer#build
-  - Hanami::Mailer#self.included
+    - Hanami::Mailer#bind
+    - Hanami::Mailer::Configuration#initialize
+    - Hanami::Mailer::Dsl#self.extended
+    - Hanami::Mailer::TemplatesFinder#find
 UncommunicativeMethodName:
   enabled: true
   reject:
@@ -104,9 +97,7 @@ UncommunicativeParameterName:
   - !ruby/regexp /[0-9]$/
   - !ruby/regexp /[A-Z]/
   accept: []
-  exclude:
-  - Hanami::Mailer#method_missing
-  - Hanami::Mailer#respond_to_missing?
+  exclude: []
 UncommunicativeVariableName:
   enabled: true
   reject:
@@ -114,10 +105,7 @@ UncommunicativeVariableName:
   - !ruby/regexp /[0-9]$/
   - !ruby/regexp /[A-Z]/
   accept: []
-  exclude:
-  - Hanami::Mailer::Configuration#duplicate
-  - Hanami::Mailer::Configuration#load!
-  - Hanami::Mailer#build
+  exclude: []
 UnusedParameters:
   enabled: true
   exclude: []
@@ -125,15 +113,10 @@ UtilityFunction:
   enabled: true
   exclude:
   - Devtools::Project::Initializer::Rspec#require_files # intentional for deduplication
-  - Hanami::Mailer::Rendering::TemplateName#tokens
   max_helper_calls: 0
 PrimaDonnaMethod:
-  exclude:
-  - Hanami::Mailer::Configuration
-  - Hanami::Mailer::Rendering::TemplateName
+  exclude: []
 ModuleInitialize:
-  exclude:
-  - Hanami::Mailer
+  exclude: []
 InstanceVariableAssumption:
-  exclude:
-  - Hanami::Mailer::Configuration
+  exclude: []
diff --git a/config/rubocop.yml b/config/rubocop.yml
deleted file mode 100644
index a0b0976..0000000
--- a/config/rubocop.yml
+++ /dev/null
@@ -1,97 +0,0 @@
-inherit_from:
-  - https://raw.githubusercontent.com/hanami/hanami/master/.rubocop.yml
-
-Metrics/BlockLength:
-  Exclude:
-  # Ignore RSpec DSL
-  - spec/**/*
-
-# Avoid parameter lists longer than four parameters.
-ParameterLists:
-  Max: 4
-  CountKeywordArgs: true
-
-# Avoid more than `Max` levels of nesting.
-BlockNesting:
-  Max: 3
-
-# Align with the style guide.
-CollectionMethods:
-  PreferredMethods:
-    collect:  'map'
-    inject:   'reduce'
-    find:     'detect'
-    find_all: 'select'
-
-# Disable documentation checking until a class needs to be documented once
-Documentation:
-  Enabled: false
-
-# Do not favor modifier if/unless usage when you have a single-line body
-IfUnlessModifier:
-  Enabled: false
-
-# Allow case equality operator (in limited use within the specs)
-CaseEquality:
-  Enabled: false
-
-# Constants do not always have to use SCREAMING_SNAKE_CASE
-ConstantName:
-  Enabled: false
-
-# Not all trivial readers/writers can be defined with attr_* methods
-TrivialAccessors:
-  Enabled: false
-
-# Allow empty lines around class body
-EmptyLinesAroundClassBody:
-  Enabled: false
-
-# Allow empty lines around module body
-EmptyLinesAroundModuleBody:
-  Enabled: false
-
-# Allow empty lines around block body
-EmptyLinesAroundBlockBody:
-  Enabled: false
-
-# Allow multiple line operations to not require indentation
-MultilineOperationIndentation:
-  Enabled: false
-
-# Prefer String#% over Kernel#sprintf
-FormatString:
-  Enabled: false
-
-# Align if/else blocks with the variable assignment
-EndAlignment:
-  EnforcedStyleAlignWith: variable
-
-# Do not always align parameters when it is easier to read
-AlignParameters:
-  Exclude:
-    - spec/**/*_spec.rb
-
-# Prefer #kind_of? over #is_a?
-ClassCheck:
-  EnforcedStyle: kind_of?
-
-# Do not prefer double quotes to be used when %q or %Q is more appropriate
-UnneededPercentQ:
-  Enabled: false
-
-# Do not prefer lambda.call(...) over lambda.(...)
-LambdaCall:
-  Enabled: false
-
-# Allow additional spaces
-ExtraSpacing:
-  Enabled: false
-
-# All objects can still be mutated if their eigenclass is patched
-RedundantFreeze:
-  Enabled: false
-
-# Prefer using `fail` when raising and `raise` when reraising
-SignalException:
-  Enabled: false
diff --git a/examples/base.rb b/examples/base.rb
new file mode 100644
index 0000000..baa485f
--- /dev/null
+++ b/examples/base.rb
@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+require 'bundler/setup'
+require 'hanami/mailer'
+
+configuration = Hanami::Mailer::Configuration.new do |config|
+  config.root            = File.expand_path(__dir__, "base")
+  config.delivery_method = :test
+end
+
+class Invoice
+  attr_reader :id, :number
+
+  def initialize(id, number)
+    @id     = id
+    @number = number
+    freeze
+  end
+end
+
+class User
+  attr_reader :name, :email
+
+  def initialize(name, email)
+    @name  = name
+    @email = email
+    freeze
+  end
+end
+
+class InvoiceMailer < Hanami::Mailer
+  template "invoice"
+
+  from "invoices@domain.test"
+  to ->(locals) { locals.fetch(:user).email }
+
+  subject ->(locals) { "Invoice ##{locals.fetch(:invoice).number}" }
+end
+
+configuration = Hanami::Mailer.finalize(configuration)
+
+invoice = Invoice.new(1, 23)
+user    = User.new("Luca", "luca@domain.test")
+
+mailer = InvoiceMailer.new(configuration: configuration)
+puts mailer.deliver(invoice: invoice, user: user)
diff --git a/examples/base/invoice.html.erb b/examples/base/invoice.html.erb
new file mode 100644
index 0000000..425171b
--- /dev/null
+++ b/examples/base/invoice.html.erb
@@ -0,0 +1,8 @@
+<html>
+  <head>
+    <title>Invoice</title>
+  </head>
+  <body>
+    <h1>Invoice #<%= invoice.number %></h1>
+  </body>
+</html>
diff --git a/examples/base/invoice.txt.erb b/examples/base/invoice.txt.erb
new file mode 100644
index 0000000..9aefc3f
--- /dev/null
+++ b/examples/base/invoice.txt.erb
@@ -0,0 +1 @@
+Invoice #<%= invoice.number %>
diff --git a/hanami-mailer.gemspec b/hanami-mailer.gemspec
index c1316f3..c43b968 100644
--- a/hanami-mailer.gemspec
+++ b/hanami-mailer.gemspec
@@ -25,5 +25,5 @@ Gem::Specification.new do |spec|
 
   spec.add_development_dependency 'bundler', '~> 1.15'
   spec.add_development_dependency 'rake',    '~> 12'
-  spec.add_development_dependency 'rspec',   '~> 3.5'
+  spec.add_development_dependency 'rspec',   '~> 3.7'
 end
diff --git a/lib/hanami-mailer.rb b/lib/hanami-mailer.rb
deleted file mode 100644
index e8132d3..0000000
--- a/lib/hanami-mailer.rb
+++ /dev/null
@@ -1 +0,0 @@
-require 'hanami/mailer' # rubocop:disable Naming/FileName
diff --git a/lib/hanami/mailer.rb b/lib/hanami/mailer.rb
index 1805d41..8fcab53 100644
--- a/lib/hanami/mailer.rb
+++ b/lib/hanami/mailer.rb
@@ -1,8 +1,7 @@
-require 'hanami/utils/class_attribute'
-require 'hanami/mailer/version'
-require 'hanami/mailer/configuration'
-require 'hanami/mailer/dsl'
+# frozen_string_literal: true
+
 require 'mail'
+require 'concurrent'
 
 # Hanami
 #
@@ -11,23 +10,12 @@ module Hanami
   # Hanami::Mailer
   #
   # @since 0.1.0
-  module Mailer
-    # Base error for Hanami::Mailer
-    #
-    # @since 0.1.0
-    class Error < ::StandardError
-    end
-
-    # Missing delivery data error
-    #
-    # It's raised when a mailer doesn't specify <tt>from</tt> or <tt>to</tt>.
-    #
-    # @since 0.1.0
-    class MissingDeliveryDataError < Error
-      def initialize
-        super("Missing delivery data, please check 'from', or 'to'")
-      end
-    end
+  class Mailer
+    require 'hanami/mailer/version'
+    require 'hanami/mailer/template'
+    require 'hanami/mailer/finalizer'
+    require 'hanami/mailer/configuration'
+    require 'hanami/mailer/dsl'
 
     # Content types mapping
     #
@@ -38,173 +26,156 @@ module Hanami
       txt:  'text/plain'
     }.freeze
 
-    include Utils::ClassAttribute
+    private_constant(:CONTENT_TYPES)
 
-    # @since 0.1.0
-    # @api private
-    class_attribute :configuration
-    self.configuration = Configuration.new
-
-    # Configure the framework.
-    # It yields the given block in the context of the configuration
-    #
-    # @param blk [Proc] the configuration block
+    # Base error for Hanami::Mailer
     #
     # @since 0.1.0
-    #
-    # @see Hanami::Mailer::Configuration
-    #
-    # @example
-    #   require 'hanami/mailer'
-    #
-    #   Hanami::Mailer.configure do
-    #     root '/path/to/root'
-    #   end
-    def self.configure(&blk)
-      configuration.instance_eval(&blk)
-      self
+    class Error < ::StandardError
     end
 
+    # Unknown mailer
+    #
+    # This error is raised at the runtime when trying to deliver a mail message,
+    # by using a configuration that it wasn't finalized yet.
+    #
+    # @since next
+    # @api unstable
+    #
+    # @see Hanami::Mailer.finalize
+    class UnknownMailerError < Error
+      # @param mailer [Hanami::Mailer] a mailer
+      #
+      # @since next
+      # @api unstable
+      def initialize(mailer)
+        super("Unknown mailer: #{mailer.inspect}. Please finalize the configuration before to use it.")
+      end
+    end
+
+    # Missing delivery data error
+    #
+    # It's raised when a mailer doesn't specify `from` or `to`.
+    #
+    # @since 0.1.0
+    class MissingDeliveryDataError < Error
+      def initialize
+        super("Missing delivery data, please check 'from', or 'to'")
+      end
+    end
+
+    # @since next
+    # @api unstable
+    @_subclasses = Concurrent::Array.new
+
     # Override Ruby's hook for modules.
-    # It includes basic Hanami::Mailer modules to the given Class.
+    # It includes basic `Hanami::Mailer` modules to the given Class.
     # It sets a copy of the framework configuration
     #
     # @param base [Class] the target mailer
     #
-    # @since 0.1.0
-    # @api private
-    #
-    # @see http://www.ruby-doc.org/core/Module.html#method-i-included
-    def self.included(base)
-      conf = configuration
-      conf.add_mailer(base)
-
-      base.class_eval do
-        extend Dsl
-        extend ClassMethods
-
-        include Utils::ClassAttribute
-        class_attribute :configuration
-
-        self.configuration = conf.duplicate
-      end
-
-      conf.copy!(base)
+    # @since next
+    # @api unstable
+    def self.inherited(base)
+      @_subclasses.push(base)
+      base.extend Dsl
     end
 
-    # Test deliveries
+    private_class_method :inherited
+
+    # Finalize the configuration
     #
-    # This is a collection of delivered messages, used when <tt>delivery_method</tt>
-    # is set on <tt>:test</tt>
+    # This should be used before to start to use the mailers
     #
-    # @return [Array] a collection of delivered messages
+    # @param configuration [Hanami::Mailer::Configuration] the configuration to
+    #   finalize
     #
-    # @since 0.1.0
+    # @return [Hanami::Mailer::Configuration] the finalized configuration
     #
-    # @see Hanami::Mailer::Configuration#delivery_mode
+    # @since next
+    # @api unstable
     #
     # @example
     #   require 'hanami/mailer'
     #
-    #   Hanami::Mailer.configure do
-    #     delivery_method :test
-    #   end.load!
+    #   configuration = Hanami::Mailer::Configuration.new do |config|
+    #     # ...
+    #   end
     #
-    #   # In testing code
-    #   Signup::Welcome.deliver
-    #   Hanami::Mailer.deliveries.count # => 1
-    def self.deliveries
-      Mail::TestMailer.deliveries
-    end
-
-    # Load the framework
-    #
-    # @since 0.1.0
-    # @api private
-    def self.load!
-      Mail.eager_autoload!
-      configuration.load!
-    end
-
-    # @since 0.1.0
-    module ClassMethods
-      # Delivers a multipart email message.
-      #
-      # When a mailer defines a <tt>html</tt> and <tt>txt</tt> template, they are
-      # both delivered.
-      #
-      # In order to selectively deliver only one of the two templates, use
-      # <tt>Signup::Welcome.deliver(format: :txt)</tt>
-      #
-      # All the given locals, excepted the reserved ones (<tt>:format</tt> and
-      # <tt>charset</tt>), are available as rendering context for the templates.
-      #
-      # @param locals [Hash] a set of objects that acts as context for the rendering
-      # @option :format [Symbol] specify format to deliver
-      # @option :charset [String] charset
-      #
-      # @since 0.1.0
-      #
-      # @see Hanami::Mailer::Configuration#default_charset
-      #
-      # @example
-      #   require 'hanami/mailer'
-      #
-      #   Hanami::Mailer.configure do
-      #     delivery_method :smtp
-      #   end.load!
-      #
-      #   module Billing
-      #     class Invoice
-      #       include Hanami::Mailer
-      #
-      #       from    'noreply@example.com'
-      #       to      :recipient
-      #       subject :subject_line
-      #
-      #       def prepare
-      #         mail.attachments['invoice.pdf'] = File.read('/path/to/invoice.pdf')
-      #       end
-      #
-      #       private
-      #
-      #       def recipient
-      #         user.email
-      #       end
-      #
-      #       def subject_line
-      #         "Invoice - #{ invoice.number }"
-      #       end
-      #     end
-      #   end
-      #
-      #   invoice = Invoice.new
-      #   user    = User.new(name: 'L', email: 'user@example.com')
-      #
-      #   Billing::Invoice.deliver(invoice: invoice, user: user)                      # Deliver both text, HTML parts and the attachment
-      #   Billing::Invoice.deliver(invoice: invoice, user: user, format: :txt)        # Deliver only the text part and the attachment
-      #   Billing::Invoice.deliver(invoice: invoice, user: user, format: :html)       # Deliver only the text part and the attachment
-      #   Billing::Invoice.deliver(invoice: invoice, user: user, charset: 'iso-8859') # Deliver both the parts with "iso-8859"
-      def deliver(locals = {})
-        new(locals).deliver
-      end
+    #   configuration = Hanami::Mailer.finalize(configuration)
+    #   MyMailer.new(configuration: configuration)
+    def self.finalize(configuration)
+      Finalizer.finalize(@_subclasses, configuration)
     end
 
     # Initialize a mailer
     #
-    # @param locals [Hash] a set of objects that acts as context for the rendering
-    # @option :format [Symbol] specify format to deliver
-    # @option :charset [String] charset
+    # @param configuration [Hanami::Mailer::Configuration] the configuration
+    # @return [Hanami::Mailer]
     #
     # @since 0.1.0
-    def initialize(locals = {})
-      @locals  = locals
-      @format  = locals.fetch(:format, nil)
-      @charset = locals.fetch(:charset, self.class.configuration.default_charset)
-      @mail    = build
-      prepare
+    def initialize(configuration:)
+      @configuration = configuration
+      freeze
     end
 
+    # Prepare the email message when a new mailer is initialized.
+    #
+    # @return [Mail::Message] the delivered email
+    #
+    # @since 0.1.0
+    # @api unstable
+    #
+    # @see Hanami::Mailer::Configuration#default_charset
+    #
+    # @example
+    #   require 'hanami/mailer'
+    #
+    #   configuration = Hanami::Mailer::Configuration.new do |config|
+    #     config.delivery_method = :smtp
+    #   end
+    #
+    #   configuration = Hanami::Mailer.finalize(configuration)
+    #
+    #   module Billing
+    #     class InvoiceMailer < Hanami::Mailer
+    #       from    'noreply@example.com'
+    #       to      ->(locals) { locals.fetch(:user).email }
+    #       subject ->(locals) { "Invoice number #{locals.fetch(:invoice).number}" }
+    #
+    #       before do |mail, locals|
+    #         mail.attachments["invoice-#{locals.fetch(:invoice).number}.pdf"] =
+    #           File.read('/path/to/invoice.pdf')
+    #       end
+    #     end
+    #   end
+    #
+    #   invoice = Invoice.new(number: 23)
+    #   user    = User.new(name: 'L', email: 'user@example.com')
+    #
+    #   mailer = Billing::InvoiceMailer.new(configuration: configuration)
+    #
+    #   # Deliver both text, HTML parts and the attachment
+    #   mailer.deliver(invoice: invoice, user: user)
+    #
+    #   # Deliver only the text part and the attachment
+    #   mailer.deliver(invoice: invoice, user: user, format: :txt)
+    #
+    #   # Deliver only the text part and the attachment
+    #   mailer.deliver(invoice: invoice, user: user, format: :html)
+    #
+    #   # Deliver both the parts with "iso-8859"
+    #   mailer.deliver(invoice: invoice, user: user, charset: 'iso-8859')
+    def deliver(locals)
+      mail(locals).deliver
+    rescue ArgumentError
+      raise MissingDeliveryDataError
+    end
+
+    # @since next
+    # @api unstable
+    alias call deliver
+
     # Render a single template with the specified format.
     #
     # @param format [Symbol] format
@@ -212,127 +183,78 @@ module Hanami
     # @return [String] the output of the rendering process.
     #
     # @since 0.1.0
-    # @api private
-    def render(format)
-      self.class.templates(format).render(self, @locals)
+    # @api unstable
+    def render(format, locals)
+      template(format).render(self, locals)
     end
 
-    # Delivers a multipart email, by looking at all the associated templates and render them.
-    #
-    # @since 0.1.0
-    # @api private
-    def deliver
-      mail.deliver
-    rescue ArgumentError => exception
-      raise MissingDeliveryDataError if exception.message =~ /SMTP (From|To) address/
-      raise
-    end
-
-    protected
-
-    # Prepare the email message when a new mailer is initialized.
-    #
-    # This is a hook that can be overwritten by mailers.
-    #
-    # @since 0.1.0
-    #
-    # @example
-    #   require 'hanami/mailer'
-    #
-    #   module Billing
-    #     class Invoice
-    #       include Hanami::Mailer
-    #
-    #       subject 'Invoice'
-    #       from    'noreply@example.com'
-    #       to      ''
-    #
-    #       def prepare
-    #         mail.attachments['invoice.pdf'] = File.read('/path/to/invoice.pdf')
-    #       end
-    #
-    #       private
-    #
-    #       def recipient
-    #         user.email
-    #       end
-    #     end
-    #   end
-    #
-    #   invoice = Invoice.new
-    #   user    = User.new(name: 'L', email: 'user@example.com')
-    def prepare
-    end
-
-    # @api private
-    # @since 0.1.0
-    def method_missing(m)
-      @locals.fetch(m) { super }
-    end
-
-    # @since 0.1.0
-    attr_reader :mail
-
-    # @api private
-    # @since 0.1.0
-    attr_reader :charset
-
     private
 
-    # rubocop:disable Metrics/MethodLength
-    # rubocop:disable Metrics/AbcSize
-    # @api private
-    def build
-      Mail.new.tap do |m|
-        m.from    = __dsl(:from)
-        m.to      = __dsl(:to)
-        m.cc      = __dsl(:cc)
-        m.bcc     = __dsl(:bcc)
-        m.subject = __dsl(:subject)
+    # @api unstable
+    # @since next
+    attr_reader :configuration
 
-        m.charset   = charset
-        m.html_part = __part(:html)
-        m.text_part = __part(:txt)
-
-        m.delivery_method(*Hanami::Mailer.configuration.delivery_method)
+    # @api unstable
+    # @since next
+    def mail(locals)
+      Mail.new.tap do |mail|
+        instance_exec(mail, locals, &self.class.before)
+        bind(mail, locals)
       end
     end
-    # rubocop:enable Metrics/MethodLength
-    # rubocop:enable Metrics/AbcSize
 
-    # @api private
+    # @api unstable
+    # @since next
+    def bind(mail, locals) # rubocop:disable Metrics/AbcSize
+      charset = locals.fetch(:charset, configuration.default_charset)
+
+      mail.from    = __dsl(:from,    locals)
+      mail.to      = __dsl(:to,      locals)
+      mail.cc      = __dsl(:cc,      locals)
+      mail.bcc     = __dsl(:bcc,     locals)
+      mail.subject = __dsl(:subject, locals)
+
+      mail.html_part = __part(:html, charset, locals)
+      mail.text_part = __part(:txt,  charset, locals)
+
+      mail.charset = charset
+      mail.delivery_method(*configuration.delivery_method)
+    end
+
+    # @since next
+    # @api unstable
+    def template(format)
+      configuration.template(self.class, format)
+    end
+
     # @since 0.1.0
-    def __dsl(method_name)
+    # @api unstable
+    def __dsl(method_name, locals)
       case result = self.class.__send__(method_name)
-      when Symbol
-        __send__(result)
+      when Proc
+        result.call(locals)
       else
         result
       end
     end
 
-    # @api private
     # @since 0.1.0
-    def __part(format)
-      return unless __part?(format)
+    # @api unstable
+    def __part(format, charset, locals)
+      return unless __part?(format, locals)
 
       Mail::Part.new.tap do |part|
         part.content_type = "#{CONTENT_TYPES.fetch(format)}; charset=#{charset}"
-        part.body         = render(format)
+        part.body         = render(format, locals)
       end
     end
 
-    # @api private
     # @since 0.1.0
-    def __part?(format)
-      @format == format ||
-        (!@format && !self.class.templates(format).nil?)
-    end
-
-    # @api private
-    # @since 0.4.0
-    def respond_to_missing?(m, _include_all)
-      @locals.key?(m)
+    # @api unstable
+    def __part?(format, locals)
+      wanted = locals.fetch(:format, nil)
+      wanted == format ||
+        (!wanted && !template(format).nil?)
     end
   end
 end
diff --git a/lib/hanami/mailer/configuration.rb b/lib/hanami/mailer/configuration.rb
index 890eb75..8633db6 100644
--- a/lib/hanami/mailer/configuration.rb
+++ b/lib/hanami/mailer/configuration.rb
@@ -1,8 +1,11 @@
-require 'set'
+# frozen_string_literal: true
+
 require 'hanami/utils/kernel'
+require 'hanami/mailer/template_name'
+require 'hanami/mailer/templates_finder'
 
 module Hanami
-  module Mailer
+  class Mailer
     # Framework configuration
     #
     # @since 0.1.0
@@ -11,7 +14,7 @@ module Hanami
       #
       # @since 0.1.0
       # @api private
-      DEFAULT_ROOT = '.'.freeze
+      DEFAULT_ROOT = '.'
 
       # Default delivery method
       #
@@ -23,24 +26,33 @@ module Hanami
       #
       # @since 0.1.0
       # @api private
-      DEFAULT_CHARSET = 'UTF-8'.freeze
+      DEFAULT_CHARSET = 'UTF-8'
 
-      # @since 0.1.0
-      # @api private
-      attr_reader :mailers
-
-      # @since 0.1.0
-      # @api private
-      attr_reader :modules
+      private_constant(*constants(false))
 
       # Initialize a configuration instance
       #
+      # @yield [config] the new initialized configuration instance
       # @return [Hanami::Mailer::Configuration] a new configuration's instance
       #
       # @since 0.1.0
+      #
+      # @example Basic Usage
+      #   require 'hanami/mailer'
+      #
+      #   configuration = Hanami::Mailer::Configuration.new do |config|
+      #     config.delivery_method :smtp, ...
+      #   end
       def initialize
-        @namespace = Object
-        reset!
+        @mailers = {}
+
+        self.namespace       = Object
+        self.root            = DEFAULT_ROOT
+        self.delivery_method = DEFAULT_DELIVERY_METHOD
+        self.default_charset = DEFAULT_CHARSET
+
+        yield(self) if block_given?
+        @finder = TemplatesFinder.new(root)
       end
 
       # Set the Ruby namespace where to lookup for mailers.
@@ -49,91 +61,52 @@ module Hanami
       # that if a `MyApp` wants a `Mailers::Signup` mailer, we are loading the
       # right one.
       #
-      # If not set, this value defaults to `Object`.
+      # @!attribute namespace
+      #   @return [Class,Module,String] the Ruby namespace where the mailers
+      #     are located
       #
-      # This is part of a DSL, for this reason when this method is called with
-      # an argument, it will set the corresponding instance variable. When
-      # called without, it will return the already set value, or the default.
+      # @since next
+      # @api unstable
       #
-      # @overload namespace(value)
-      #   Sets the given value
-      #   @param value [Class, Module, String] a valid Ruby namespace identifier
-      #
-      # @overload namespace
-      #   Gets the value
-      #   @return [Class, Module, String]
-      #
-      # @api private
-      # @since 0.1.0
-      #
-      # @example Getting the value
+      # @example
       #   require 'hanami/mailer'
       #
-      #   Hanami::Mailer.configuration.namespace # => Object
-      #
-      # @example Setting the value
-      #   require 'hanami/mailer'
-      #
-      #   Hanami::Mailer.configure do
-      #     namespace 'MyApp::Mailers'
+      #   Hanami::Mailer::Configuration.new do |config|
+      #     config.namespace = MyApp::Mailers
       #   end
-      def namespace(value = nil)
-        if value
-          @namespace = value
-        else
-          @namespace
-        end
-      end
+      attr_accessor :namespace
 
       # Set the root path where to search for templates
       #
       # If not set, this value defaults to the current directory.
       #
-      # When this method is called with an argument, it will set the corresponding instance variable.
-      # When called without, it will return the already set value, or the default.
+      # @param value [String, Pathname] the root path for mailer templates
       #
-      # @overload root(value)
-      #   Sets the given value
-      #   @param value [String, Pathname, #to_pathname] an object that can be
-      #     coerced to Pathname
+      # @raise [Errno::ENOENT] if the path doesn't exist
       #
-      # @overload root
-      #   Gets the value
-      #   @return [Pathname]
-      #
-      # @since 0.1.0
+      # @since next
+      # @api unstable
       #
       # @see http://www.ruby-doc.org/stdlib/libdoc/pathname/rdoc/Pathname.html
       # @see http://rdoc.info/gems/hanami-utils/Hanami/Utils/Kernel#Pathname-class_method
       #
-      # @example Getting the value
+      # @example
       #   require 'hanami/mailer'
       #
-      #   Hanami::Mailer.configuration.root # => #<Pathname:.>
-      #
-      # @example Setting the value
-      #   require 'hanami/mailer'
-      #
-      #   Hanami::Mailer.configure do
-      #     root '/path/to/templates'
+      #   Hanami::Mailer::Configuration.new do |config|
+      #     config.root = 'path/to/templates'
       #   end
-      #
-      #   Hanami::Mailer.configuration.root # => #<Pathname:/path/to/templates>
-      def root(value = nil)
-        if value
-          @root = Utils::Kernel.Pathname(value).realpath
-        else
-          @root
-        end
+      def root=(value)
+        @root = Utils::Kernel.Pathname(value).realpath
       end
 
-      # Prepare the mailers.
-      #
-      # The given block will be yielded when `Hanami::Mailer` will be included by
-      # a mailer.
-      #
-      # This method can be called multiple times.
+      # @!attribute [r] root
+      #   @return [Pathname] the root path for mailer templates
       #
+      # @since next
+      # @api unstable
+      attr_reader :root
+
       # @param blk [Proc] the code block
       #
       # @return [void]
@@ -144,19 +117,8 @@ module Hanami
       #
       # @see Hanami::Mailer.configure
       def prepare(&blk)
-        if block_given? # rubocop:disable Style/GuardClause
-          @modules.push(blk)
-        else
-          raise ArgumentError.new('Please provide a block')
-        end
-      end
-
-      # Add a mailer to the registry
-      #
-      # @since 0.1.0
-      # @api private
-      def add_mailer(mailer)
-        @mailers.add(mailer)
+        raise ArgumentError.new('Please provide a block') unless block_given?
+        @modules.push(blk)
       end
 
       # Duplicate by copying the settings in a new instance.
@@ -211,40 +173,41 @@ module Hanami
       #
       # It supports the following delivery methods:
       #
-      #   * Exim (<tt>:exim</tt>)
-      #   * Sendmail (<tt>:sendmail</tt>)
-      #   * SMTP (<tt>:smtp</tt>, for local installations)
-      #   * SMTP Connection (<tt>:smtp_connection</tt>,
-      #     via <tt>Net::SMTP</tt> - for remote installations)
-      #   * Test (<tt>:test</tt>, for testing purposes)
+      #   * Exim (`:exim`)
+      #   * Sendmail (`:sendmail`)
+      #   * SMTP (`:smtp`, for local installations)
+      #   * SMTP Connection (`:smtp_connection`,
+      #     via `Net::SMTP` - for remote installations)
+      #   * Test (`:test`, for testing purposes)
       #
-      # The default delivery method is SMTP (<tt>:smtp</tt>).
+      # The default delivery method is SMTP (`:smtp`).
       #
       # Custom delivery methods can be specified by passing the class policy and
       # a set of optional configurations. This class MUST respond to:
       #
-      #   * <tt>initialize(options = {})</tt>
-      #   * <tt>deliver!(mail)<tt>
+      #   * `initialize(options = {})`
+      #   * `deliver!(mail)`
       #
       # @param method [Symbol, #initialize, deliver!] delivery method
       # @param options [Hash] optional settings
       #
       # @return [Array] an array containing the delivery method and the optional settings as an Hash
       #
-      # @since 0.1.0
+      # @since next
+      # @api unstable
       #
       # @example Setup delivery method with supported symbol
       #   require 'hanami/mailer'
       #
-      #   Hanami::Mailer.configure do
-      #     delivery_method :sendmail
+      #   Hanami::Mailer::Configuration.new do |config|
+      #     config.delivery_method = :sendmail
       #   end
       #
       # @example Setup delivery method with supported symbol and options
       #   require 'hanami/mailer'
       #
-      #   Hanami::Mailer.configure do
-      #     delivery_method :smtp, address: "localhost", port: 1025
+      #   Hanami::Mailer::Configuration.new do |config|
+      #     config.delivery_method = :smtp, address: "localhost", port: 1025
       #   end
       #
       # @example Setup custom delivery method with options
@@ -260,49 +223,77 @@ module Hanami
       #     end
       #   end
       #
-      #   Hanami::Mailer.configure do
-      #     delivery_method MandrillDeliveryMethod,
-      #       username: ENV['MANDRILL_USERNAME'],
-      #       password: ENV['MANDRILL_API_KEY']
+      #   Hanami::Mailer.Configuration.new do |config|
+      #     config.delivery_method = MandrillDeliveryMethod,
+      #                                username: ENV['MANDRILL_USERNAME'],
+      #                                password: ENV['MANDRILL_API_KEY']
       #   end
-      def delivery_method(method = nil, options = {})
-        if method.nil?
-          @delivery_method
-        else
-          @delivery_method = [method, options]
-        end
+      attr_accessor :delivery_method
+
+      # Specify a default charset for all the delivered emails
+      #
+      # If not set, it defaults to `UTF-8`
+      #
+      # @!attribute default_charset
+      #   @return [String] the charset
+      #
+      # @since next
+      # @api unstable
+      #
+      # @example
+      #   require 'hanami/mailer'
+      #
+      #   Hanami::Mailer::Configuration.new do |config|
+      #     config.default_charset = "iso-8859-1"
+      #   end
+      attr_accessor :default_charset
+
+      # Add a mailer to the registry
+      #
+      # @param mailer [Hanami::Mailer] a mailer
+      #
+      # @since 0.1.0
+      # @api unstable
+      def add_mailer(mailer)
+        template_name = TemplateName[mailer.template_name, namespace]
+        templates     = finder.find(template_name)
+
+        mailers[mailer] = templates
       end
 
-      # @since 0.1.0
-      def default_charset(value = nil)
-        if value.nil?
-          @default_charset
-        else
-          @default_charset = value
-        end
+      # @param mailer [Hanami::Mailer] a mailer
+      # @param format [Symbol] the wanted format (eg. `:html`, `:txt`)
+      #
+      # @raise [Hanami::Mailer::UnknownMailerError] if the given mailer is not
+      #   present in the configuration. This happens when the configuration is
+      #   used before to being finalized.
+      #
+      # @since next
+      # @api unstable
+      def template(mailer, format)
+        mailers.fetch(mailer) { raise UnknownMailerError.new(mailer) }[format]
       end
 
-      protected
+      # Deep freeze the important instance variables
+      #
+      # @since next
+      # @api unstable
+      def freeze
+        delivery_method.freeze
+        default_charset.freeze
+        mailers.freeze
+        super
+      end
 
-      # @api private
-      # @since 0.1.0
-      attr_writer :root
+      private
 
-      # @api private
       # @since 0.1.0
-      attr_writer :delivery_method
+      # @api private
+      attr_reader :mailers
 
-      # @api private
-      # @since 0.1.0
-      attr_writer :default_charset
-
-      # @api private
-      # @since 0.1.0
-      attr_writer :namespace
-
-      # @api private
-      # @since 0.1.0
-      attr_writer :modules
+      # @since next
+      # @api unstable
+      attr_reader :finder
     end
   end
 end
diff --git a/lib/hanami/mailer/dsl.rb b/lib/hanami/mailer/dsl.rb
index fd8e91f..ba09f54 100644
--- a/lib/hanami/mailer/dsl.rb
+++ b/lib/hanami/mailer/dsl.rb
@@ -1,89 +1,31 @@
-require 'hanami/mailer/rendering/template_name'
-require 'hanami/mailer/rendering/templates_finder'
+# frozen_string_literal: true
 
 module Hanami
-  module Mailer
+  # Hanami::Mailer
+  #
+  # @since 0.1.0
+  class Mailer
+    require 'hanami/mailer/template_name'
+
     # Class level DSL
     #
     # @since 0.1.0
     module Dsl
       # @since 0.3.0
-      # @api private
+      # @api unstable
       def self.extended(base)
         base.class_eval do
-          @from    = nil
-          @to      = nil
-          @cc      = nil
-          @bcc     = nil
-          @subject = nil
+          @from     = nil
+          @to       = nil
+          @cc       = nil
+          @bcc      = nil
+          @subject  = nil
+          @template = nil
+          @before   = ->(*) {}
         end
       end
 
-      # Set the template name IF it differs from the convention.
-      #
-      # For a given mailer named <tt>Signup::Welcome</tt> it will look for
-      # <tt>signup/welcome.*.*</tt> templates under the root directory.
-      #
-      # If for some reason, we need to specify a different template name, we can
-      # use this method.
-      #
-      # This is part of a DSL, for this reason when this method is called with
-      # an argument, it will set the corresponding class variable. When
-      # called without, it will return the already set value, or the default.
-      #
-      # @overload template(value)
-      #   Sets the given value
-      #   @param value [String, #to_s] relative template path, under root
-      #   @return [NilClass]
-      #
-      # @overload template
-      #   Gets the template name
-      #   @return [String]
-      #
-      # @since 0.1.0
-      #
-      # @see Hanami::Mailers::Configuration.root
-      #
-      # @example Custom template name
-      #   require 'hanami/mailer'
-      #
-      #   class MyMailer
-      #     include Hanami::Mailer
-      #     template 'mailer'
-      #   end
-      def template(value = nil)
-        if value.nil?
-          @template ||= ::Hanami::Mailer::Rendering::TemplateName.new(name, configuration.namespace).to_s
-        else
-          @template = value
-        end
-      end
-
-      # Returns a set of associated templates or only one for the given format
-      #
-      # This is part of a DSL, for this reason when this method is called with
-      # an argument, it will set the corresponding class variable. When
-      # called without, it will return the already set value, or the default.
-      #
-      # @overload templates(format)
-      #   Returns the template associated with the given format
-      #   @param value [Symbol] the format
-      #   @return [Hash]
-      #
-      # @overload templates
-      #   Returns all the associated templates
-      #   Gets the template name
-      #   @return [Hash] a set of templates
-      #
-      # @since 0.1.0
-      # @api private
-      def templates(format = nil)
-        if format.nil?
-          @templates = ::Hanami::Mailer::Rendering::TemplatesFinder.new(self).find
-        else
-          @templates.fetch(format, nil)
-        end
-      end
+      private_class_method :extended
 
       # Sets the sender for mail messages
       #
@@ -114,24 +56,15 @@ module Hanami
       # @example Hardcoded value (String)
       #   require 'hanami/mailer'
       #
-      #   class WelcomeMailer
-      #     include Hanami::Mailer
-      #
+      #   class WelcomeMailer < Hanami::Mailer
       #     from "noreply@example.com"
       #   end
       #
-      # @example Method (Symbol)
+      # @example Lazy (Proc)
       #   require 'hanami/mailer'
       #
-      #   class WelcomeMailer
-      #     include Hanami::Mailer
-      #     from :sender
-      #
-      #     private
-      #
-      #     def sender
-      #       "noreply@example.com"
-      #     end
+      #   class WelcomeMailer < Hanami::Mailer
+      #     from ->(locals) { locals.fetch(:sender).email }
       #   end
       def from(value = nil)
         if value.nil?
@@ -141,6 +74,74 @@ module Hanami
         end
       end
 
+      # Sets the recipient for mail messages
+      #
+      # It accepts a hardcoded value as a string or array of strings.
+      # For dynamic values, you can specify a symbol that represents an instance
+      # method.
+      #
+      # This value MUST be set, otherwise an exception is raised at the delivery
+      # time.
+      #
+      # When a value is given, specify the recipient of the email
+      # Otherwise, it returns the recipient of the email
+      #
+      # This is part of a DSL, for this reason when this method is called with
+      # an argument, it will set the corresponding class variable. When
+      # called without, it will return the already set value, or the default.
+      #
+      # @overload to(value)
+      #   Sets the recipient
+      #   @param value [String, Array, Symbol] the hardcoded value or method name
+      #   @return [NilClass]
+      #
+      # @overload to
+      #   Returns the recipient
+      #   @return [String, Array, Symbol] the recipient
+      #
+      # @since 0.1.0
+      #
+      # @example Hardcoded value (String)
+      #   require 'hanami/mailer'
+      #
+      #   class WelcomeMailer < Hanami::Mailer
+      #     to "user@example.com"
+      #   end
+      #
+      # @example Hardcoded value (Array)
+      #   require 'hanami/mailer'
+      #
+      #   class WelcomeMailer < Hanami::Mailer
+      #     to ["user-1@example.com", "user-2@example.com"]
+      #   end
+      #
+      # @example Lazy value (Proc)
+      #   require 'hanami/mailer'
+      #
+      #   class WelcomeMailer < Hanami::Mailer
+      #     to ->(locals) { locals.fetch(:user).email }
+      #   end
+      #
+      #   user = User.new(name: 'L')
+      #   WelcomeMailer.new(configuration: configuration).deliver(user: user)
+      #
+      # @example Lazy values (Proc)
+      #   require 'hanami/mailer'
+      #
+      #   class WelcomeMailer < Hanami::Mailer
+      #     to ->(locals) { locals.fetch(:users).map(&:email) }
+      #   end
+      #
+      #   users = [User.new(name: 'L'), User.new(name: 'MG')]
+      #   WelcomeMailer.new(configuration: configuration).deliver(users: users)
+      def to(value = nil)
+        if value.nil?
+          @to
+        else
+          @to = value
+        end
+      end
+
       # Sets the cc (carbon copy) for mail messages
       #
       # It accepts a hardcoded value as a string or array of strings.
@@ -170,58 +171,36 @@ module Hanami
       # @example Hardcoded value (String)
       #   require 'hanami/mailer'
       #
-      #   class WelcomeMailer
-      #     include Hanami::Mailer
-      #
-      #     to "user@example.com"
+      #   class WelcomeMailer < Hanami::Mailer
       #     cc "other.user@example.com"
       #   end
       #
       # @example Hardcoded value (Array)
       #   require 'hanami/mailer'
       #
-      #   class WelcomeMailer
-      #     include Hanami::Mailer
-      #
-      #     to ["user-1@example.com", "user-2@example.com"]
+      #   class WelcomeMailer < Hanami::Mailer
       #     cc ["other.user-1@example.com", "other.user-2@example.com"]
       #   end
       #
-      # @example Method (Symbol)
+      # @example Lazy value (Proc)
       #   require 'hanami/mailer'
       #
-      #   class WelcomeMailer
-      #     include Hanami::Mailer
-      #     to "user@example.com"
-      #     cc :email_address
-      #
-      #     private
-      #
-      #     def email_address
-      #       user.email
-      #     end
+      #   class WelcomeMailer < Hanami::Mailer
+      #     cc ->(locals) { locals.fetch(:user).email }
       #   end
       #
-      #   other_user = User.new(name: 'L')
-      #   WelcomeMailer.deliver(user: other_user)
+      #   user = User.new(name: 'L')
+      #   WelcomeMailer.new(configuration: configuration).deliver(user: user)
       #
-      # @example Method that returns a collection of recipients
+      # @example Lazy values (Proc)
       #   require 'hanami/mailer'
       #
-      #   class WelcomeMailer
-      #     include Hanami::Mailer
-      #     to "user@example.com"
-      #     cc :recipients
-      #
-      #     private
-      #
-      #     def recipients
-      #       users.map(&:email)
-      #     end
+      #   class WelcomeMailer < Hanami::Mailer
+      #     cc ->(locals) { locals.fetch(:users).map(&:email) }
       #   end
       #
-      #   other_users = [User.new(name: 'L'), User.new(name: 'MG')]
-      #   WelcomeMailer.deliver(users: other_users)
+      #   users = [User.new(name: 'L'), User.new(name: 'MG')]
+      #   WelcomeMailer.new(configuration: configuration).deliver(users: users)
       def cc(value = nil)
         if value.nil?
           @cc
@@ -259,58 +238,36 @@ module Hanami
       # @example Hardcoded value (String)
       #   require 'hanami/mailer'
       #
-      #   class WelcomeMailer
-      #     include Hanami::Mailer
-      #
-      #     to "user@example.com"
+      #   class WelcomeMailer < Hanami::Mailer
       #     bcc "other.user@example.com"
       #   end
       #
       # @example Hardcoded value (Array)
       #   require 'hanami/mailer'
       #
-      #   class WelcomeMailer
-      #     include Hanami::Mailer
-      #
-      #     to ["user-1@example.com", "user-2@example.com"]
+      #   class WelcomeMailer < Hanami::Mailer
       #     bcc ["other.user-1@example.com", "other.user-2@example.com"]
       #   end
       #
-      # @example Method (Symbol)
+      # @example Lazy value (Proc)
       #   require 'hanami/mailer'
       #
-      #   class WelcomeMailer
-      #     include Hanami::Mailer
-      #     to "user@example.com"
-      #     bcc :email_address
-      #
-      #     private
-      #
-      #     def email_address
-      #       user.email
-      #     end
+      #   class WelcomeMailer < Hanami::Mailer
+      #     bcc ->(locals) { locals.fetch(:user).email }
       #   end
       #
-      #   other_user = User.new(name: 'L')
-      #   WelcomeMailer.deliver(user: other_user)
+      #   user = User.new(name: 'L')
+      #   WelcomeMailer.new(configuration: configuration).deliver(user: user)
       #
-      # @example Method that returns a collection of recipients
+      # @example Lazy values (Proc)
       #   require 'hanami/mailer'
       #
-      #   class WelcomeMailer
-      #     include Hanami::Mailer
-      #     to "user@example.com"
-      #     bcc :recipients
-      #
-      #     private
-      #
-      #     def recipients
-      #       users.map(&:email)
-      #     end
+      #   class WelcomeMailer < Hanami::Mailer
+      #     bcc ->(locals) { locals.fetch(:users).map(&:email) }
       #   end
       #
-      #   other_users = [User.new(name: 'L'), User.new(name: 'MG')]
-      #   WelcomeMailer.deliver(users: other_users)
+      #   users = [User.new(name: 'L'), User.new(name: 'MG')]
+      #   WelcomeMailer.new(configuration: configuration).deliver(users: users)
       def bcc(value = nil)
         if value.nil?
           @bcc
@@ -319,92 +276,6 @@ module Hanami
         end
       end
 
-      # Sets the recipient for mail messages
-      #
-      # It accepts a hardcoded value as a string or array of strings.
-      # For dynamic values, you can specify a symbol that represents an instance
-      # method.
-      #
-      # This value MUST be set, otherwise an exception is raised at the delivery
-      # time.
-      #
-      # When a value is given, specify the recipient of the email
-      # Otherwise, it returns the recipient of the email
-      #
-      # This is part of a DSL, for this reason when this method is called with
-      # an argument, it will set the corresponding class variable. When
-      # called without, it will return the already set value, or the default.
-      #
-      # @overload to(value)
-      #   Sets the recipient
-      #   @param value [String, Array, Symbol] the hardcoded value or method name
-      #   @return [NilClass]
-      #
-      # @overload to
-      #   Returns the recipient
-      #   @return [String, Array, Symbol] the recipient
-      #
-      # @since 0.1.0
-      #
-      # @example Hardcoded value (String)
-      #   require 'hanami/mailer'
-      #
-      #   class WelcomeMailer
-      #     include Hanami::Mailer
-      #
-      #     to "user@example.com"
-      #   end
-      #
-      # @example Hardcoded value (Array)
-      #   require 'hanami/mailer'
-      #
-      #   class WelcomeMailer
-      #     include Hanami::Mailer
-      #
-      #     to ["user-1@example.com", "user-2@example.com"]
-      #   end
-      #
-      # @example Method (Symbol)
-      #   require 'hanami/mailer'
-      #
-      #   class WelcomeMailer
-      #     include Hanami::Mailer
-      #     to :email_address
-      #
-      #     private
-      #
-      #     def email_address
-      #       user.email
-      #     end
-      #   end
-      #
-      #   user = User.new(name: 'L')
-      #   WelcomeMailer.deliver(user: user)
-      #
-      # @example Method that returns a collection of recipients
-      #   require 'hanami/mailer'
-      #
-      #   class WelcomeMailer
-      #     include Hanami::Mailer
-      #     to :recipients
-      #
-      #     private
-      #
-      #     def recipients
-      #       users.map(&:email)
-      #     end
-      #   end
-      #
-      #   users = [User.new(name: 'L'), User.new(name: 'MG')]
-      #   WelcomeMailer.deliver(users: users)
-      def to(value = nil)
-        if value.nil?
-          @to
-        else
-          @to = value
-        end
-      end
-
       # Sets the subject for mail messages
       #
       # It accepts a hardcoded value as a string, or a symbol that represents
@@ -431,28 +302,19 @@ module Hanami
       # @example Hardcoded value (String)
       #   require 'hanami/mailer'
       #
-      #   class WelcomeMailer
-      #     include Hanami::Mailer
-      #
+      #   class WelcomeMailer < Hanami::Mailer
       #     subject "Welcome"
       #   end
       #
-      # @example Method (Symbol)
+      # @example Lazy value (Proc)
       #   require 'hanami/mailer'
       #
-      #   class WelcomeMailer
-      #     include Hanami::Mailer
-      #     subject :greeting
-      #
-      #     private
-      #
-      #     def greeting
-      #       "Hello, #{ user.name }"
-      #     end
+      #   class WelcomeMailer < Hanami::Mailer
+      #     subject ->(locals) { "Hello #{locals.fetch(:user).name}" }
       #   end
       #
       #   user = User.new(name: 'L')
-      #   WelcomeMailer.deliver(user: user)
+      #   WelcomeMailer.new(configuration: configuration).deliver(user: user)
       def subject(value = nil)
         if value.nil?
           @subject
@@ -461,17 +323,70 @@ module Hanami
         end
       end
 
-      protected
-
-      # Loading mechanism hook.
+      # Set the template name **IF** it differs from the naming convention.
+      #
+      # For a given mailer named `Signup::Welcome` it will look for
+      # `signup/welcome.*.*` templates under the root directory.
+      #
+      # If for some reason, we need to specify a different template name, we can
+      # use this method.
+      #
+      # @param value [String] the template name
       #
-      # @api private
       # @since 0.1.0
+      # @api unstable
       #
-      # @see Hanami::Mailer.load!
-      def load!
-        templates.freeze
-        configuration.freeze
+      # @example Custom template name
+      #   require 'hanami/mailer'
+      #
+      #   class MyMailer < Hanami::Mailer
+      #     template 'mailer'
+      #   end
+      def template(value)
+        @template = value
+      end
+
+      # @since next
+      # @api unstable
+      def template_name
+        @template || name
+      end
+
+      # Before callback for email delivery
+      #
+      # @since next
+      # @api unstable
+      #
+      # @example
+      #   require 'hanami/mailer'
+      #
+      #   module Billing
+      #     class InvoiceMailer < Hanami::Mailer
+      #       subject 'Invoice'
+      #       from    'noreply@example.com'
+      #       to      ->(locals) { locals.fetch(:user).email }
+      #
+      #       before do |mail, locals|
+      #         user = locals.fetch(:user)
+      #         mail.attachments["invoice-#{invoice_code}-#{user.id}.pdf"] = File.read('/path/to/invoice.pdf')
+      #       end
+      #
+      #       def invoice_code
+      #         "123"
+      #       end
+      #     end
+      #   end
+      #
+      #   invoice = Invoice.new
+      #   user    = User.new(name: 'L', email: 'user@example.com')
+      #
+      #   InvoiceMailer.new(configuration: configuration).deliver(invoice: invoice, user: user)
+      def before(&blk)
+        if block_given?
+          @before = blk
+        else
+          @before
+        end
       end
     end
   end
diff --git a/lib/hanami/mailer/finalizer.rb b/lib/hanami/mailer/finalizer.rb
new file mode 100644
index 0000000..c257ab1
--- /dev/null
+++ b/lib/hanami/mailer/finalizer.rb
@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+require 'mail'
+# require 'ice_nine'
+
+module Hanami
+  class Mailer
+    # @since next
+    # @api unstable
+    class Finalizer
+      # Finalize the given configuration before to start to use the mailers
+      #
+      # @param mailers [Array<Hanami::Mailer>] all the subclasses of
+      #   `Hanami::Mailer`
+      # @param configuration [Hanami::Mailer::Configuration] the configuration
+      #   to finalize
+      #
+      # @return configuration [Hanami::Mailer::Configuration] the finalized
+      #   configuration
+      #
+      # @since next
+      # @api unstable
+      def self.finalize(mailers, configuration)
+        Mail.eager_autoload!
+        mailers.each { |mailer| configuration.add_mailer(mailer) }
+
+        configuration.freeze
+        configuration
+      end
+    end
+  end
+end
diff --git a/lib/hanami/mailer/rendering/template_name.rb b/lib/hanami/mailer/rendering/template_name.rb
deleted file mode 100644
index e7abb82..0000000
--- a/lib/hanami/mailer/rendering/template_name.rb
+++ /dev/null
@@ -1,53 +0,0 @@
-require 'hanami/utils/string'
-
-module Hanami
-  module Mailer
-    module Rendering
-      # @since 0.1.0
-      # @api private
-      #
-      # TODO this is identical to Hanami::View, consider to move into Hanami::Utils
-      class TemplateName
-        # @since 0.1.0
-        # @api private
-        NAMESPACE_SEPARATOR = '::'.freeze
-
-        # @since 0.1.0
-        # @api private
-        def initialize(name, namespace)
-          @name = name
-          compile!(namespace)
-        end
-
-        # @since 0.1.0
-        # @api private
-        def to_s
-          @name
-        end
-
-        private
-
-        # @since 0.1.0
-        # @api private
-        def compile!(namespace)
-          tokens(namespace) { |token| replace!(token) }
-          @name = Utils::String.underscore(@name)
-        end
-
-        # @since 0.1.0
-        # @api private
-        def tokens(namespace)
-          namespace.to_s.split(NAMESPACE_SEPARATOR).each do |token|
-            yield token
-          end
-        end
-
-        # @since 0.1.0
-        # @api private
-        def replace!(token)
-          @name.gsub!(/\A#{token}#{NAMESPACE_SEPARATOR}/, '')
-        end
-      end
-    end
-  end
-end
diff --git a/lib/hanami/mailer/rendering/templates_finder.rb b/lib/hanami/mailer/rendering/templates_finder.rb
deleted file mode 100644
index ef0ebcc..0000000
--- a/lib/hanami/mailer/rendering/templates_finder.rb
+++ /dev/null
@@ -1,133 +0,0 @@
-require 'hanami/mailer/template'
-
-module Hanami
-  module Mailer
-    module Rendering
-      # Find templates for a mailer
-      #
-      # @api private
-      # @since 0.1.0
-      #
-      # @see Mailer::Template
-      class TemplatesFinder
-        # Default format
-        #
-        # @api private
-        # @since 0.1.0
-        FORMAT    = '*'.freeze
-
-        # Default template engines
-        #
-        # @api private
-        # @since 0.1.0
-        ENGINES   = '*'.freeze
-
-        # Recursive pattern
-        #
-        # @api private
-        # @since 0.1.0
-        RECURSIVE = '**'.freeze
-
-        # Initialize a finder
-        #
-        # @param mailer [Class] the mailer class
-        #
-        # @api private
-        # @since 0.1.0
-        def initialize(mailer)
-          @mailer = mailer
-        end
-
-        # Find all the associated templates to the mailer.
-        # It recursively looks for templates under the root path of the mailer,
-        # that are matching the template name
-        #
-        # @return [Hash] the templates
-        #
-        # @api private
-        # @since 0.1.0
-        #
-        # @see Hanami::Mailer::Dsl#root
-        # @see Hanami::Mailer::Dsl#templates
-        #
-        # @example
-        #   require 'hanami/mailer'
-        #
-        #   module Mailers
-        #     class Welcome
-        #       include Hanami::Mailer
-        #     end
-        #   end
-        #
-        #   Mailers::Welcome.root     # => "/path/to/templates"
-        #   Mailers::Welcome.templates # => {[:html] => "welcome"}
-        #
-        #   # This mailer has a template:
-        #   #
-        #   #   "/path/to/templates/welcome.html.erb"
-        #
-        #   Hanami::Mailer::Rendering::TemplatesFinder.new(Mailers::Welcome).find
-        #     # => [#<Hanami::Mailer::Template:0x007f8a0a86a970 ... @file="/path/to/templates/welcome.html.erb">]
-        def find
-          templates = Hash[]
-          _find.map do |template|
-            name = File.basename(template)
-            format = (name.split('.')[-2]).to_sym
-            templates[format] = Mailer::Template.new(template)
-          end
-          templates
-        end
-
-        protected
-
-        # @api private
-        # @since 0.1.0
-        def _find(lookup = search_path)
-          Dir.glob("#{[root, lookup, template_name].join(separator)}.#{format}.#{engines}")
-        end
-
-        # @api private
-        # @since 0.1.0
-        def template_name
-          Rendering::TemplateName.new(@mailer.template, @mailer.configuration.namespace).to_s
-        end
-
-        # @api private
-        # @since 0.1.0
-        def root
-          @mailer.configuration.root
-        end
-
-        # @api private
-        # @since 0.1.0
-        def search_path
-          recursive
-        end
-
-        # @api private
-        # @since 0.1.0
-        def recursive
-          RECURSIVE
-        end
-
-        # @api private
-        # @since 0.1.0
-        def separator
-          ::File::SEPARATOR
-        end
-
-        # @api private
-        # @since 0.1.0
-        def format
-          FORMAT
-        end
-
-        # @api private
-        # @since 0.1.0
-        def engines
-          ENGINES
-        end
-      end
-    end
-  end
-end
diff --git a/lib/hanami/mailer/template.rb b/lib/hanami/mailer/template.rb
index f328a1a..e0857e0 100644
--- a/lib/hanami/mailer/template.rb
+++ b/lib/hanami/mailer/template.rb
@@ -1,7 +1,9 @@
+# frozen_string_literal: true
+
 require 'tilt'
 
 module Hanami
-  module Mailer
+  class Mailer
     # A logic-less template.
     #
     # @api private
@@ -11,29 +13,20 @@ module Hanami
     class Template
       def initialize(template)
         @_template = Tilt.new(template)
+        freeze
       end
 
       # Render the template within the context of the given scope.
       #
-      # @param scope [Class] the rendering scope
+      # @param scope [Object] the rendering scope
       # @param locals [Hash] set of objects passed to the constructor
       #
       # @return [String] the output of the rendering process
       #
       # @api private
       # @since 0.1.0
-      def render(scope = Object.new, locals = {})
-        @_template.render(scope, locals)
-      end
-
-      # Get the path to the template
-      #
-      # @return [String] the pathname
-      #
-      # @api private
-      # @since 0.1.0
-      def file
-        @_template.file
+      def render(scope, locals = {})
+        @_template.render(scope.dup, locals)
       end
     end
   end
diff --git a/lib/hanami/mailer/template_name.rb b/lib/hanami/mailer/template_name.rb
new file mode 100644
index 0000000..ba47938
--- /dev/null
+++ b/lib/hanami/mailer/template_name.rb
@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+require 'hanami/utils/string'
+
+module Hanami
+  class Mailer
+    # @since 0.1.0
+    # @api private
+    #
+    # TODO this is identical to Hanami::View, consider to move into Hanami::Utils
+    class TemplateName
+      # @since next
+      # @api unstable
+      def self.call(name, namespace)
+        Utils::String.new(name.gsub(/\A#{namespace}(::)*/, '')).underscore.to_s
+      end
+
+      class << self
+        # @since next
+        # @api unstable
+        alias [] call
+      end
+    end
+  end
+end
diff --git a/lib/hanami/mailer/templates_finder.rb b/lib/hanami/mailer/templates_finder.rb
new file mode 100644
index 0000000..d199a53
--- /dev/null
+++ b/lib/hanami/mailer/templates_finder.rb
@@ -0,0 +1,152 @@
+# frozen_string_literal: true
+
+require 'hanami/mailer/template'
+require 'hanami/utils/file_list'
+require 'pathname'
+
+module Hanami
+  class Mailer
+    # Find templates for a mailer
+    #
+    # @api private
+    # @since 0.1.0
+    #
+    # @see Mailer::Template
+    class TemplatesFinder
+      # Default format
+      #
+      # @api private
+      # @since 0.1.0
+      FORMAT    = '*'
+
+      # Default template engines
+      #
+      # @api private
+      # @since 0.1.0
+      ENGINES   = '*'
+
+      # Recursive pattern
+      #
+      # @api private
+      # @since 0.1.0
+      RECURSIVE = '**'
+
+      # Format separator
+      #
+      # @api unstable
+      # @since next
+      #
+      # @example
+      #   welcome.html.erb
+      FORMAT_SEPARATOR = '.'
+
+      private_constant(*constants(true))
+
+      # Initialize a finder
+      #
+      # @param root [String,Pathname] the root directory where to recursively
+      #   look for templates
+      #
+      # @raise [Errno::ENOENT] if the directory doesn't exist
+      #
+      # @api unstable
+      # @since 0.1.0
+      def initialize(root)
+        @root = Pathname.new(root).realpath
+        freeze
+      end
+
+      # Find all the associated templates to the mailer.
+      #
+      # It starts under the root path and it **recursively** looks for templates
+      # that are matching the given template name.
+      #
+      # @param template_name [String] the template name
+      #
+      # @return [Hash] the templates
+      #
+      # @api unstable
+      # @since 0.1.0
+      #
+      # @example
+      #   require 'hanami/mailer'
+      #
+      #   module Mailers
+      #     class Welcome < Hanami::Mailer
+      #     end
+      #   end
+      #
+      #   configuration = Hanami::Mailer::Configuration.new do |config|
+      #     config.root = "path/to/templates"
+      #   end
+      #
+      #   # This mailer has a template:
+      #   #
+      #   #   "path/to/templates/welcome.html.erb"
+      #
+      #   Hanami::Mailer::Rendering::TemplatesFinder.new(root).find("welcome")
+      #     # => [#<Hanami::Mailer::Template:0x007f8a0a86a970 ... @file="path/to/templates/welcome.html.erb">]
+      def find(template_name)
+        templates(template_name).each_with_object({}) do |template, result|
+          format         = extract_format(template)
+          result[format] = Mailer::Template.new(template)
+        end
+      end
+
+      protected
+
+      # @api unstable
+      # @since 0.1.0
+      def templates(template_name, lookup = search_path)
+        Utils::FileList["#{[root, lookup, template_name].join(separator)}#{format_separator}#{format}#{format_separator}#{engines}"]
+      end
+
+      # @api unstable
+      # @since 0.1.0
+      attr_reader :root
+
+      # @api private
+      # @since 0.1.0
+      def search_path
+        recursive
+      end
+
+      # @api private
+      # @since 0.1.0
+      def recursive
+        RECURSIVE
+      end
+
+      # @api private
+      # @since 0.1.0
+      def separator
+        ::File::SEPARATOR
+      end
+
+      # @api private
+      # @since 0.1.0
+      def format
+        FORMAT
+      end
+
+      # @api private
+      # @since 0.1.0
+      def engines
+        ENGINES
+      end
+
+      # @api unstable
+      # @since next
+      def format_separator
+        FORMAT_SEPARATOR
+      end
+
+      # @api unstable
+      # @since next
+      def extract_format(template)
+        filename = File.basename(template)
+        filename.split(format_separator)[-2].to_sym
+      end
+    end
+  end
+end
diff --git a/lib/hanami/mailer/version.rb b/lib/hanami/mailer/version.rb
index 06b0719..0dc5806 100644
--- a/lib/hanami/mailer/version.rb
+++ b/lib/hanami/mailer/version.rb
@@ -1,6 +1,8 @@
+# frozen_string_literal: true
+
 module Hanami
-  module Mailer
+  class Mailer
     # @since 0.1.0
-    VERSION = '1.1.0'.freeze
+    VERSION = '1.1.0'
   end
 end
diff --git a/spec/integration/hanami/mailer/delivery_spec.rb b/spec/integration/hanami/mailer/delivery_spec.rb
new file mode 100644
index 0000000..e8603d1
--- /dev/null
+++ b/spec/integration/hanami/mailer/delivery_spec.rb
@@ -0,0 +1,154 @@
+# frozen_string_literal: true
+
+RSpec.describe Hanami::Mailer do
+  describe '.deliver' do
+    it 'can deliver with specified charset' do
+      mail = CharsetMailer.new(configuration: configuration).deliver(charset: charset = 'iso-2022-jp')
+
+      expect(mail.charset).to             eq(charset)
+      expect(mail.parts.first.charset).to eq(charset)
+    end
+
+    it "raises error when 'from' isn't specified" do
+      expect { MissingFromMailer.new(configuration: configuration).deliver({}) }.to raise_error(Hanami::Mailer::MissingDeliveryDataError)
+    end
+
+    it "raises error when 'to' isn't specified" do
+      expect { MissingToMailer.new(configuration: configuration).deliver({}) }.to raise_error(Hanami::Mailer::MissingDeliveryDataError)
+    end
+
+    describe 'test delivery with hardcoded values' do
+      subject { WelcomeMailer.new(configuration: configuration).deliver({}) }
+
+      it 'sends the correct information' do
+        expect(subject.from).to    eq(['noreply@sender.com'])
+        expect(subject.to).to      eq(['noreply@recipient.com', 'owner@recipient.com'])
+        expect(subject.cc).to      eq(['cc@recipient.com'])
+        expect(subject.bcc).to     eq(['bcc@recipient.com'])
+        expect(subject.subject).to eq('Welcome')
+      end
+
+      it 'has the correct templates' do
+        expect(subject.html_part.to_s).to include(%(template))
+        expect(subject.text_part.to_s).to include(%(template))
+      end
+
+      it 'interprets the prepare statement' do
+        attachment = subject.attachments['invoice.pdf']
+
+        expect(attachment).to be_kind_of(Mail::Part)
+
+        expect(attachment).to be_attachment
+        expect(attachment).to_not be_inline
+        expect(attachment).to_not be_multipart
+
+        expect(attachment.filename).to eq('invoice.pdf')
+
+        expect(attachment.content_type).to match('application/pdf')
+        expect(attachment.content_type).to match('filename=invoice.pdf')
+      end
+    end
+
+    describe 'test delivery with procs' do
+      subject { ProcMailer.new(configuration: configuration).deliver(user: user) }
+      let(:user) { User.new('Name', 'student@deigirls.com') }
+
+      it 'sends the correct information' do
+        expect(subject.from).to    eq(["hello-#{user.name.downcase}@example.com"])
+        expect(subject.to).to      eq([user.email])
+        expect(subject.subject).to eq("[Hanami] Hello, #{user.name}")
+      end
+    end
+
+    describe 'test delivery with locals' do
+      subject     { EventMailer.new(configuration: configuration) }
+      let(:count) { 100 }
+
+      it 'delivers the message' do
+        threads = []
+        mails   = {}
+
+        count.times do |i|
+          threads << Thread.new do
+            user  = OpenStruct.new(name: "Luca #{i}", email: "luca-#{i}@domain.test")
+            event = OpenStruct.new(id: i, title: "Event ##{i}")
+
+            mails[i] = subject.deliver(user: user, event: event)
+          end
+        end
+        threads.map(&:join)
+
+        expect(mails.count).to eq(count)
+        mails.each do |i, mail|
+          expect(mail.to).to                      eq(["luca-#{i}@domain.test"])
+          expect(mail.subject).to                 eq("Invitation: Event ##{i}")
+          expect(mail.attachments[0].filename).to eq("invitation-#{i}.ics")
+        end
+      end
+    end
+
+    describe 'multipart' do
+      it 'delivers all the parts by default' do
+        mail = WelcomeMailer.new(configuration: configuration).deliver({})
+        body = mail.body.encoded
+
+        expect(body).to include(%(<h1>Hello World!</h1>))
+        expect(body).to include(%(This is a txt template))
+      end
+
+      it 'can deliver only the text part' do
+        mail = WelcomeMailer.new(configuration: configuration).deliver(format: :txt)
+        body = mail.body.encoded
+
+        expect(body).to_not include(%(<h1>Hello World!</h1>))
+        expect(body).to     include(%(This is a txt template))
+      end
+
+      it 'can deliver only the html part' do
+        mail = WelcomeMailer.new(configuration: configuration).deliver(format: :html)
+        body = mail.body.encoded
+
+        expect(body).to     include(%(<h1>Hello World!</h1>))
+        expect(body).to_not include(%(This is a txt template))
+      end
+    end
+
+    describe 'custom delivery' do
+      before do
+        mailer.deliver({})
+      end
+
+      subject       { options.fetch(:deliveries).first }
+      let(:mailer)  { WelcomeMailer.new(configuration: configuration) }
+      let(:options) { { deliveries: [] } }
+
+      let(:configuration) do
+        configuration = Hanami::Mailer::Configuration.new do |config|
+          config.root            = 'spec/support/fixtures'
+          config.delivery_method = MandrillDeliveryMethod, options
+        end
+
+        Hanami::Mailer.finalize(configuration)
+      end
+
+      it 'delivers the mail' do
+        expect(options.fetch(:deliveries).size).to be(1)
+      end
+
+      it 'sends the correct information' do
+        expect(subject.from).to    eq(['noreply@sender.com'])
+        expect(subject.to).to      eq(['noreply@recipient.com', 'owner@recipient.com'])
+        expect(subject.subject).to eq("Welcome")
+      end
+
+      it 'has the correct templates' do
+        expect(subject.html_part.to_s).to include(%(template))
+        expect(subject.text_part.to_s).to include(%(template))
+      end
+
+      it 'runs the before callback' do
+        expect(subject.attachments['invoice.pdf']).to_not be(nil)
+      end
+    end
+  end
+end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index f28edbc..57942ae 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 RSpec.configure do |config|
   config.expect_with :rspec do |expectations|
     expectations.include_chain_clauses_in_custom_matcher_descriptions = true
@@ -22,35 +24,9 @@ RSpec.configure do |config|
   Kernel.srand config.seed
 end
 
+require 'ostruct'
 require 'hanami/utils'
 
 $LOAD_PATH.unshift 'lib'
 require 'hanami/mailer'
-
-Hanami::Mailer.configure do
-  root Pathname.new __dir__ + '/support/fixtures/templates'
-end
-
-Hanami::Mailer.class_eval do
-  def self.reset!
-    self.configuration = configuration.duplicate
-    configuration.reset!
-  end
-end
-
-Hanami::Mailer::Dsl.class_eval do
-  def reset!
-    @templates = {}
-  end
-end
-
 Hanami::Utils.require!("spec/support")
-
-Hanami::Mailer.configure do
-  root __dir__ + '/support/fixtures'
-  delivery_method :test
-
-  prepare do
-    include DefaultSubject
-  end
-end.load!
diff --git a/spec/support/context.rb b/spec/support/context.rb
new file mode 100644
index 0000000..d4cd077
--- /dev/null
+++ b/spec/support/context.rb
@@ -0,0 +1,22 @@
+module RSpec
+  module Support
+    module Context
+      def self.included(base)
+        base.class_eval do
+          let(:configuration) do
+            configuration = Hanami::Mailer::Configuration.new do |config|
+              config.root            = 'spec/support/fixtures'
+              config.delivery_method = :test
+            end
+
+            Hanami::Mailer.finalize(configuration)
+          end
+        end
+      end
+    end
+  end
+end
+
+RSpec.configure do |config|
+  config.include(RSpec::Support::Context)
+end
diff --git a/spec/support/fixtures.rb b/spec/support/fixtures.rb
index 0eb38f0..5971fda 100644
--- a/spec/support/fixtures.rb
+++ b/spec/support/fixtures.rb
@@ -1,42 +1,36 @@
-class InvoiceMailer
-  include Hanami::Mailer
+# frozen_string_literal: true
+
+class InvoiceMailer < Hanami::Mailer
   template 'invoice'
 end
 
-class RenderMailer
-  include Hanami::Mailer
+class RenderMailer < Hanami::Mailer
 end
 
-class TemplateEngineMailer
-  include Hanami::Mailer
+class TemplateEngineMailer < Hanami::Mailer
 end
 
-class CharsetMailer
-  include Hanami::Mailer
-
+class CharsetMailer < Hanami::Mailer
   from    'noreply@example.com'
   to      'user@example.com'
   subject 'こんにちは'
 end
 
-class MissingFromMailer
-  include Hanami::Mailer
+class MissingFromMailer < Hanami::Mailer
   template 'missing'
 
   to      'recipient@example.com'
   subject 'Hello'
 end
 
-class MissingToMailer
-  include Hanami::Mailer
+class MissingToMailer < Hanami::Mailer
   template 'missing'
 
   from    'sender@example.com'
   subject 'Hello'
 end
 
-class CcOnlyMailer
-  include Hanami::Mailer
+class CcOnlyMailer < Hanami::Mailer
   template 'missing'
 
   cc      'recipient@example.com'
@@ -44,8 +38,7 @@ class CcOnlyMailer
   subject 'Hello'
 end
 
-class BccOnlyMailer
-  include Hanami::Mailer
+class BccOnlyMailer < Hanami::Mailer
   template 'missing'
 
   bcc      'recipient@example.com'
@@ -55,35 +48,20 @@ end
 
 User = Struct.new(:name, :email)
 
-class LazyMailer
-  include Hanami::Mailer
+class LazyMailer < Hanami::Mailer
 end
 
-class MethodMailer
-  include Hanami::Mailer
+class ProcMailer < Hanami::Mailer
+  from    ->(locals) { "hello-#{locals.fetch(:user).name.downcase}@example.com" }
+  to      ->(locals) { locals.fetch(:user).email }
+  subject ->(locals) { "[Hanami] #{locals.fetch(:greeting)}" }
 
-  from    :sender
-  to      :recipient
-  subject :greeting
-
-  def greeting
-    "Hello, #{user.name}"
-  end
-
-  private
-
-  def sender
-    "hello-#{user.name.downcase}@example.com"
-  end
-
-  def recipient
-    user.email
+  before do |_, locals|
+    locals[:greeting] = "Hello, #{locals.fetch(:user).name}"
   end
 end
 
-class WelcomeMailer
-  include Hanami::Mailer
-
+class WelcomeMailer < Hanami::Mailer
   from 'noreply@sender.com'
   to   ['noreply@recipient.com', 'owner@recipient.com']
   cc   'cc@recipient.com'
@@ -91,12 +69,34 @@ class WelcomeMailer
 
   subject 'Welcome'
 
+  before do |mail|
+    mail.attachments['invoice.pdf'] = "/path/to/invoice-#{invoice_code}.pdf"
+  end
+
   def greeting
     'Ahoy'
   end
 
-  def prepare
-    mail.attachments['invoice.pdf'] = '/path/to/invoice.pdf'
+  def invoice_code
+    "123"
+  end
+end
+
+class EventMailer < Hanami::Mailer
+  from    'events@domain.test'
+  to      ->(locals) { locals.fetch(:user).email }
+  subject ->(locals) { "Invitation: #{locals.fetch(:event).title}" }
+
+  before do |mail, locals|
+    mail.attachments["invitation-#{locals.fetch(:event).id}.ics"] = generate_invitation_attachment(locals)
+  end
+
+  private
+
+  # Simulate on-the-fly creation of an attachment file.
+  # For speed purposes we're not gonna create the file, but only return a path.
+  def generate_invitation_attachment(locals)
+    "invitation-#{locals.fetch(:event).id}.ics"
   end
 end
 
@@ -111,8 +111,14 @@ class MandrillDeliveryMethod
 end
 
 module Users
-  class Welcome
-    include Hanami::Mailer
+  class Welcome < Hanami::Mailer
+  end
+end
+
+module Web
+  module Mailers
+    class SignupMailer < Hanami::Mailer
+    end
   end
 end
 
diff --git a/spec/support/fixtures/templates/method_mailer.txt.erb b/spec/support/fixtures/templates/proc_mailer.txt.erb
similarity index 100%
rename from spec/support/fixtures/templates/method_mailer.txt.erb
rename to spec/support/fixtures/templates/proc_mailer.txt.erb
diff --git a/spec/support/fixtures/templates/welcome_mailer b/spec/support/fixtures/templates/welcome_mailer
new file mode 100644
index 0000000..97be962
--- /dev/null
+++ b/spec/support/fixtures/templates/welcome_mailer
@@ -0,0 +1,2 @@
+Fixture used by
+spec/unit/hanami/mailer/templates_finder_spec.rb
diff --git a/spec/support/fixtures/templates/welcome_mailer.erb b/spec/support/fixtures/templates/welcome_mailer.erb
new file mode 100644
index 0000000..97be962
--- /dev/null
+++ b/spec/support/fixtures/templates/welcome_mailer.erb
@@ -0,0 +1,2 @@
+Fixture used by
+spec/unit/hanami/mailer/templates_finder_spec.rb
diff --git a/spec/support/fixtures/templates/welcome_mailer.html b/spec/support/fixtures/templates/welcome_mailer.html
new file mode 100644
index 0000000..97be962
--- /dev/null
+++ b/spec/support/fixtures/templates/welcome_mailer.html
@@ -0,0 +1,2 @@
+Fixture used by
+spec/unit/hanami/mailer/templates_finder_spec.rb
diff --git a/spec/unit/hanami/mailer/configuration_spec.rb b/spec/unit/hanami/mailer/configuration_spec.rb
index 50e3f4f..0d5b903 100644
--- a/spec/unit/hanami/mailer/configuration_spec.rb
+++ b/spec/unit/hanami/mailer/configuration_spec.rb
@@ -1,21 +1,21 @@
-RSpec.describe Hanami::Mailer::Configuration do
-  before do
-    @configuration = Hanami::Mailer::Configuration.new
-  end
+# frozen_string_literal: true
 
-  describe '#root' do
+RSpec.describe Hanami::Mailer::Configuration do
+  subject { described_class.new }
+
+  describe '#root=' do
     describe 'when a value is given' do
       describe 'and it is a string' do
         it 'sets it as a Pathname' do
-          @configuration.root 'spec'
-          expect(@configuration.root).to eq(Pathname.new('spec').realpath)
+          subject.root = 'spec'
+          expect(subject.root).to eq(Pathname.new('spec').realpath)
         end
       end
 
       describe 'and it is a pathname' do
         it 'sets it' do
-          @configuration.root Pathname.new('spec')
-          expect(@configuration.root).to eq(Pathname.new('spec').realpath)
+          subject.root = Pathname.new('spec')
+          expect(subject.root).to eq(Pathname.new('spec').realpath)
         end
       end
 
@@ -33,15 +33,15 @@ RSpec.describe Hanami::Mailer::Configuration do
         end
 
         it 'sets the converted value' do
-          @configuration.root RootPath.new('spec')
-          expect(@configuration.root).to eq(Pathname.new('spec').realpath)
+          subject.root = RootPath.new('spec')
+          expect(subject.root).to eq(Pathname.new('spec').realpath)
         end
       end
 
       describe 'and it is an unexisting path' do
         it 'raises an error' do
           expect do
-            @configuration.root '/path/to/unknown'
+            subject.root = '/path/to/unknown'
           end.to raise_error(Errno::ENOENT)
         end
       end
@@ -49,146 +49,69 @@ RSpec.describe Hanami::Mailer::Configuration do
 
     describe 'when a value is not given' do
       it 'defaults to the current path' do
-        expect(@configuration.root).to eq(Pathname.new('.').realpath)
+        expect(subject.root).to eq(Pathname.new('.').realpath)
       end
     end
   end
 
-  describe '#mailers' do
-    it 'defaults to an empty set' do
-      expect(@configuration.mailers).to be_empty
-    end
-
-    it 'allows to add mailers' do
-      @configuration.add_mailer(InvoiceMailer)
-      expect(@configuration.mailers).to include(InvoiceMailer)
-    end
-
-    it 'eliminates duplications' do
-      @configuration.add_mailer(RenderMailer)
-      @configuration.add_mailer(RenderMailer)
-
-      expect(@configuration.mailers.size).to eq(1)
-    end
-  end
-
-  describe '#prepare' do
-    before do
-      module FooRendering
-        def render
-          'foo'
-        end
-      end
-
-      class PrepareMailer
-      end
-    end
-
-    after do
-      Object.__send__(:remove_const, :FooRendering)
-      Object.__send__(:remove_const, :PrepareMailer)
-    end
-
-    it 'raises error in case of missing block' do
-      expect { @configuration.prepare }.to raise_error(ArgumentError, 'Please provide a block')
-    end
-  end
-
-  # describe '#reset!' do
-  #   before do
-  #     @configuration.root 'test'
-  #     @configuration.add_mailer(InvoiceMailer)
-
-  #     @configuration.reset!
-  #   end
-
-  #   it 'resets root' do
-  #     root = Pathname.new('.').realpath
-
-  #     @configuration.root.must_equal root
-  #     @configuration.mailers.must_be_empty
-  #   end
-
-  #   it "doesn't reset namespace" do
-  #     @configuration.namespace(InvoiceMailer)
-  #     @configuration.reset!
-
-  #     @configuration.namespace.must_equal(InvoiceMailer)
-  #   end
-
-  # end
-
-  describe '#load!' do
-    before do
-      @configuration.root 'spec'
-      @configuration.load!
-    end
-
-    it 'loads root' do
-      root = Pathname.new('spec').realpath
-      expect(@configuration.root).to eq(root)
-    end
-  end
-
   describe '#delivery_method' do
     describe 'when not previously set' do
-      before do
-        @configuration.reset!
-      end
-
       it 'defaults to SMTP' do
-        expect(@configuration.delivery_method).to eq([:smtp, {}])
+        expect(subject.delivery_method).to eq(:smtp)
       end
     end
 
     describe 'set with a symbol' do
       before do
-        @configuration.delivery_method :exim, location: '/path/to/exim'
+        subject.delivery_method = :exim, { location: '/path/to/exim' }
       end
 
       it 'saves the delivery method in the configuration' do
-        expect(@configuration.delivery_method).to eq([:exim, { location: '/path/to/exim' }])
+        expect(subject.delivery_method).to eq([:exim, { location: '/path/to/exim' }])
       end
     end
 
     describe 'set with a class' do
       before do
-        @configuration.delivery_method MandrillDeliveryMethod,
-                                       username: 'mandrill-username', password: 'mandrill-api-key'
+        subject.delivery_method = MandrillDeliveryMethod,
+                                  { username: 'mandrill-username', password: 'mandrill-api-key' }
       end
 
       it 'saves the delivery method in the configuration' do
-        expect(@configuration.delivery_method).to eq([MandrillDeliveryMethod, username: 'mandrill-username', password: 'mandrill-api-key'])
+        expect(subject.delivery_method).to eq([MandrillDeliveryMethod, username: 'mandrill-username', password: 'mandrill-api-key'])
       end
     end
   end
 
   describe '#default_charset' do
     describe 'when not previously set' do
-      before do
-        @configuration.reset!
-      end
-
       it 'defaults to UTF-8' do
-        expect(@configuration.default_charset).to eq('UTF-8')
+        expect(subject.default_charset).to eq('UTF-8')
       end
     end
 
     describe 'when set' do
       before do
-        @configuration.default_charset 'iso-8859-1'
+        subject.default_charset = 'iso-8859-1'
       end
 
       it 'saves the delivery method in the configuration' do
-        expect(@configuration.default_charset).to eq('iso-8859-1')
+        expect(subject.default_charset).to eq('iso-8859-1')
       end
     end
   end
 
-  describe '#prepare' do
-    it 'injects code in each mailer'
-    # it 'injects code in each mailer' do
-    #   InvoiceMailer.subject.must_equal 'default subject'
-    # end
+  describe "#freeze" do
+    before do
+      subject.freeze
+    end
+
+    it "is frozen" do
+      expect(subject).to be_frozen
+    end
+
+    it "raises error if trying to add a mailer" do
+      expect { subject.add_mailer(WelcomeMailer) }.to raise_error(RuntimeError)
+    end
   end
 end
diff --git a/spec/unit/hanami/mailer/delivery_spec.rb b/spec/unit/hanami/mailer/delivery_spec.rb
deleted file mode 100644
index 82cbad4..0000000
--- a/spec/unit/hanami/mailer/delivery_spec.rb
+++ /dev/null
@@ -1,184 +0,0 @@
-RSpec.describe Hanami::Mailer do
-  describe '.deliver' do
-    before do
-      Hanami::Mailer.deliveries.clear
-    end
-
-    it 'can deliver with specified charset' do
-      CharsetMailer.deliver(charset: charset = 'iso-2022-jp')
-
-      mail = Hanami::Mailer.deliveries.first
-      expect(mail.charset).to             eq(charset)
-      expect(mail.parts.first.charset).to eq(charset)
-    end
-
-    it "raises error when 'from' isn't specified" do
-      expect { MissingFromMailer.deliver }.to raise_error(Hanami::Mailer::MissingDeliveryDataError)
-    end
-
-    it "raises error when 'to' isn't specified" do
-      expect { MissingToMailer.deliver }.to raise_error(Hanami::Mailer::MissingDeliveryDataError)
-    end
-
-    it "doesn't raise error when 'cc' is specified but 'to' isn't" do
-      CcOnlyMailer.deliver
-    end
-
-    it "doesn't raise error when 'bcc' is specified but 'to' isn't" do
-      BccOnlyMailer.deliver
-    end
-
-    it "lets other errors to bubble up" do
-      mailer = CharsetMailer.new({})
-      mail   = Class.new do
-        def deliver
-          raise ArgumentError, "ouch"
-        end
-      end.new
-
-      expect(mailer).to receive(:mail).and_return(mail)
-      expect { mailer.deliver }.to raise_error(ArgumentError, "ouch")
-    end
-
-    describe 'test delivery with hardcoded values' do
-      before do
-        WelcomeMailer.deliver
-        @mail = Hanami::Mailer.deliveries.first
-      end
-
-      after do
-        Hanami::Mailer.deliveries.clear
-      end
-
-      it 'delivers the mail' do
-        expect(Hanami::Mailer.deliveries.length).to eq(1)
-      end
-
-      it 'sends the correct information' do
-        expect(@mail.from).to    eq(['noreply@sender.com'])
-        expect(@mail.to).to      eq(['noreply@recipient.com', 'owner@recipient.com'])
-        expect(@mail.cc).to      eq(['cc@recipient.com'])
-        expect(@mail.bcc).to     eq(['bcc@recipient.com'])
-        expect(@mail.subject).to eq('Welcome')
-      end
-
-      it 'has the correct templates' do
-        expect(@mail.html_part.to_s).to include(%(template))
-        expect(@mail.text_part.to_s).to include(%(template))
-      end
-
-      it 'interprets the prepare statement' do
-        attachment = @mail.attachments['invoice.pdf']
-
-        expect(attachment).to be_kind_of(Mail::Part)
-
-        expect(attachment).to be_attachment
-        expect(attachment).to_not be_inline
-        expect(attachment).to_not be_multipart
-
-        expect(attachment.filename).to eq('invoice.pdf')
-
-        expect(attachment.content_type).to match('application/pdf')
-        expect(attachment.content_type).to match('filename=invoice.pdf')
-      end
-    end
-
-    describe 'test delivery with methods' do
-      before do
-        @user = User.new('Name', 'student@deigirls.com')
-        MethodMailer.deliver(user: @user)
-
-        @mail = Hanami::Mailer.deliveries.first
-      end
-
-      after do
-        Hanami::Mailer.deliveries.clear
-      end
-
-      it 'delivers the mail' do
-        expect(Hanami::Mailer.deliveries.length).to eq(1)
-      end
-
-      it 'sends the correct information' do
-        expect(@mail.from).to    eq(["hello-#{@user.name.downcase}@example.com"])
-        expect(@mail.to).to      eq([@user.email])
-        expect(@mail.subject).to eq("Hello, #{@user.name}")
-      end
-    end
-
-    describe 'multipart' do
-      after do
-        Hanami::Mailer.deliveries.clear
-      end
-
-      it 'delivers all the parts by default' do
-        WelcomeMailer.deliver
-
-        mail = Hanami::Mailer.deliveries.first
-        body = mail.body.encoded
-
-        expect(body).to include(%(<h1>Hello World!</h1>))
-        expect(body).to include(%(This is a txt template))
-      end
-
-      it 'can deliver only the text part' do
-        WelcomeMailer.deliver(format: :txt)
-
-        mail = Hanami::Mailer.deliveries.first
-        body = mail.body.encoded
-
-        expect(body).to_not include(%(<h1>Hello World!</h1>))
-        expect(body).to     include(%(This is a txt template))
-      end
-
-      it 'can deliver only the html part' do
-        WelcomeMailer.deliver(format: :html)
-
-        mail = Hanami::Mailer.deliveries.first
-        body = mail.body.encoded
-
-        expect(body).to     include(%(<h1>Hello World!</h1>))
-        expect(body).to_not include(%(This is a txt template))
-      end
-    end
-
-    describe 'custom delivery' do
-      before do
-        @options = options = { deliveries: [] }
-
-        # Hanami::Mailer.reset!
-        # Hanami::Mailer.configure do
-        #   delivery_method MandrillDeliveryMethod, options
-        # end.load!
-
-        WelcomeMailer.deliver
-
-        @mail = options.fetch(:deliveries).first
-      end
-
-      it 'delivers the mail'
-      # it 'delivers the mail' do
-      #   @options.fetch(:deliveries).length.must_equal 1
-      # end
-
-      # it 'sends the correct information' do
-      #   @mail.from.must_equal ['noreply@sender.com']
-      #   @mail.to.must_equal ['noreply@recipient.com']
-      #   @mail.subject.must_equal "Welcome"
-      # end
-
-      # it 'has the correct templates' do
-      #   @mail.html_part.to_s.must_include %(template)
-      #   @mail.text_part.to_s.must_include %(template)
-      # end
-
-      # it 'interprets the prepare statement' do
-      #   @mail.attachments['invoice.pdf'].wont_be_nil
-      # end
-
-      # it 'adds the attachment to the mail object' do
-      #   @mail.attachments['render_mailer.html.erb'].wont_be_nil
-      # end
-    end
-  end
-end
diff --git a/spec/unit/hanami/mailer/dsl_spec.rb b/spec/unit/hanami/mailer/dsl_spec.rb
index 1e0451c..469baa0 100644
--- a/spec/unit/hanami/mailer/dsl_spec.rb
+++ b/spec/unit/hanami/mailer/dsl_spec.rb
@@ -1,47 +1,123 @@
-RSpec.describe Hanami::Mailer do
-  describe '.template' do
-    describe 'when no value is set' do
-      it 'returns the convention name' do
-        expect(RenderMailer.template).to eq('render_mailer')
-      end
+# frozen_string_literal: true
 
-      it 'returns correct namespaced value' do
-        expect(Users::Welcome.template).to eq('users/welcome')
-      end
+RSpec.describe Hanami::Mailer::Dsl do
+  let(:mailer) { Class.new { extend Hanami::Mailer::Dsl } }
+
+  describe '.from' do
+    it 'returns the default value' do
+      expect(mailer.from).to be(nil)
     end
 
-    describe 'when a value is set' do
-      it 'returns that name' do
-        expect(InvoiceMailer.template).to eq('invoice')
-      end
+    it 'sets the value' do
+      sender = 'sender@hanami.test'
+      mailer.from sender
+
+      expect(mailer.from).to eq(sender)
     end
   end
 
-  describe '.templates' do
-    describe 'when no value is set' do
-      it 'returns a set of templates' do
-        template_formats = LazyMailer.templates.keys
-        expect(template_formats).to eq(%i[html txt])
-      end
-
-      it 'returns only the template for the given format' do
-        template = LazyMailer.templates(:txt)
-        expect(template).to be_kind_of(Hanami::Mailer::Template)
-        expect(template.file).to match(%r{spec/support/fixtures/templates/lazy_mailer.txt.erb\z})
-      end
+  describe '.to' do
+    it 'returns the default value' do
+      expect(mailer.to).to be(nil)
     end
 
-    describe 'when a value is set' do
-      it 'returns a set of templates' do
-        template_formats = InvoiceMailer.templates.keys
-        expect(template_formats).to eq([:html])
-      end
+    it 'sets a single value' do
+      recipient = 'recipient@hanami.test'
+      mailer.to recipient
 
-      it 'returns only the template for the given format' do
-        template = InvoiceMailer.templates(:html)
-        expect(template).to be_kind_of(Hanami::Mailer::Template)
-        expect(template.file).to match(%r{spec/support/fixtures/templates/invoice.html.erb\z})
-      end
+      expect(mailer.to).to eq(recipient)
+    end
+
+    it 'sets an array of values' do
+      recipients = ['recipient@hanami.test']
+      mailer.to recipients
+
+      expect(mailer.to).to eq(recipients)
+    end
+  end
+
+  describe '.cc' do
+    it 'returns the default value' do
+      expect(mailer.cc).to be(nil)
+    end
+
+    it 'sets a single value' do
+      recipient = 'cc@hanami.test'
+      mailer.cc recipient
+
+      expect(mailer.cc).to eq(recipient)
+    end
+
+    it 'sets an array of values' do
+      recipients = ['cc@hanami.test']
+      mailer.cc recipients
+
+      expect(mailer.cc).to eq(recipients)
+    end
+  end
+
+  describe '.bcc' do
+    it 'returns the default value' do
+      expect(mailer.bcc).to be(nil)
+    end
+
+    it 'sets a single value' do
+      recipient = 'bcc@hanami.test'
+      mailer.bcc recipient
+
+      expect(mailer.bcc).to eq(recipient)
+    end
+
+    it 'sets an array of values' do
+      recipients = ['bcc@hanami.test']
+      mailer.bcc recipients
+
+      expect(mailer.bcc).to eq(recipients)
+    end
+  end
+
+  describe '.subject' do
+    it 'returns the default value' do
+      expect(mailer.subject).to be(nil)
+    end
+
+    it 'sets a value' do
+      mail_subject = 'Hello'
+      mailer.subject mail_subject
+
+      expect(mailer.subject).to eq(mail_subject)
+    end
+  end
+
+  describe '.template' do
+    it 'sets a value' do
+      mailer.template 'file'
+    end
+  end
+
+  describe '.template_name' do
+    it 'returns the default value' do
+      expect(mailer.template_name).to be(nil)
+    end
+
+    it 'returns value, if set' do
+      template = 'file'
+      mailer.template template
+
+      expect(mailer.template_name).to eq(template)
+    end
+  end
+
+  describe '.before' do
+    it 'returns the default value' do
+      expect(mailer.before).to be_kind_of(Proc)
+    end
+
+    it 'sets a value' do
+      blk = ->(*) {}
+      mailer.before(&blk)
+
+      expect(mailer.before).to eq(blk)
     end
   end
 end
diff --git a/spec/unit/hanami/mailer/error_spec.rb b/spec/unit/hanami/mailer/error_spec.rb
new file mode 100644
index 0000000..a8bad56
--- /dev/null
+++ b/spec/unit/hanami/mailer/error_spec.rb
@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+RSpec.describe Hanami::Mailer::Error do
+  it "inherits from StandardError" do
+    expect(described_class.ancestors).to include(StandardError)
+  end
+end
diff --git a/spec/unit/hanami/mailer/finalizer_spec.rb b/spec/unit/hanami/mailer/finalizer_spec.rb
new file mode 100644
index 0000000..84612e9
--- /dev/null
+++ b/spec/unit/hanami/mailer/finalizer_spec.rb
@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+RSpec.describe Hanami::Mailer::Finalizer do
+  let(:configuration) do
+    Hanami::Mailer::Configuration.new do |config|
+      config.root = 'spec/support/fixtures'
+    end
+  end
+
+  let(:mailers) { [double('mailer', template_name: "invoice")] }
+
+  describe '.finalize' do
+    it 'eager autoloads modules from mail gem' do
+      expect(Mail).to receive(:eager_autoload!)
+      described_class.finalize(mailers, configuration)
+    end
+
+    it "adds the mailer to the configuration" do
+      expect(configuration).to receive(:add_mailer).with(mailers.first)
+      described_class.finalize(mailers, configuration)
+    end
+
+    it "returns frozen configuration" do
+      actual = described_class.finalize(mailers, configuration)
+      expect(actual).to be_frozen
+    end
+  end
+end
diff --git a/spec/unit/hanami/mailer/missing_delivery_data_error_spec.rb b/spec/unit/hanami/mailer/missing_delivery_data_error_spec.rb
new file mode 100644
index 0000000..915fdd4
--- /dev/null
+++ b/spec/unit/hanami/mailer/missing_delivery_data_error_spec.rb
@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+RSpec.describe Hanami::Mailer::MissingDeliveryDataError do
+  it "inherits from Hanami::Error" do
+    expect(described_class.ancestors).to include(Hanami::Mailer::Error)
+  end
+
+  it "has a custom error message" do
+    expect { raise described_class }.to raise_error(described_class, "Missing delivery data, please check 'from', or 'to'")
+  end
+end
diff --git a/spec/unit/hanami/mailer/rendering_spec.rb b/spec/unit/hanami/mailer/rendering_spec.rb
deleted file mode 100644
index 6fd3811..0000000
--- a/spec/unit/hanami/mailer/rendering_spec.rb
+++ /dev/null
@@ -1,44 +0,0 @@
-RSpec.describe Hanami::Mailer do
-  describe '#render' do
-    describe 'when template is explicitly declared' do
-      let(:mailer) { InvoiceMailer.new }
-
-      it 'renders the given template' do
-        expect(mailer.render(:html)).to include(%(<h1>Invoice template</h1>))
-      end
-    end
-
-    describe 'when template is implicitly declared' do
-      let(:mailer) { LazyMailer.new }
-
-      it 'looks for template with same name with inflected classname and render it' do
-        expect(mailer.render(:html)).to include(%(Hello World))
-        expect(mailer.render(:txt)).to include(%(This is a txt template))
-      end
-    end
-
-    describe 'when mailer defines context' do
-      let(:mailer) { WelcomeMailer.new }
-
-      it 'renders template with defined context' do
-        expect(mailer.render(:txt)).to include(%(Ahoy))
-      end
-    end
-
-    describe 'when locals are parsed in' do
-      let(:mailer) { RenderMailer.new(user: User.new('Luca')) }
-
-      it 'renders template with parsed locals' do
-        expect(mailer.render(:html)).to include(%(Luca))
-      end
-    end
-
-    describe 'with HAML template engine' do
-      let(:mailer) { TemplateEngineMailer.new(user: User.new('Luca')) }
-
-      it 'renders template with parsed locals' do
-        expect(mailer.render(:html)).to include(%(<h1>\nLuca\n</h1>\n))
-      end
-    end
-  end
-end
diff --git a/spec/unit/hanami/mailer/template_name_spec.rb b/spec/unit/hanami/mailer/template_name_spec.rb
new file mode 100644
index 0000000..bfcb2eb
--- /dev/null
+++ b/spec/unit/hanami/mailer/template_name_spec.rb
@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+
+RSpec.describe Hanami::Mailer::TemplateName do
+  describe ".call" do
+    context "with top level namespace" do
+      let(:namespace) { Object }
+
+      it "returns an instance of ::String" do
+        template_name = described_class.call("template", namespace)
+        expect(template_name).to be_kind_of(String)
+      end
+
+      it "returns name from plain name" do
+        template_name = described_class.call("template", namespace)
+        expect(template_name).to eq("template")
+      end
+
+      it "returns name from camel case name" do
+        template_name = described_class.call("ATemplate", namespace)
+        expect(template_name).to eq("a_template")
+      end
+
+      it "returns name from snake case name" do
+        template_name = described_class.call("a_template", namespace)
+        expect(template_name).to eq("a_template")
+      end
+
+      it "returns name from modulized name" do
+        template_name = described_class.call("Mailers::WelcomeMailer", namespace)
+        expect(template_name).to eq("mailers/welcome_mailer")
+      end
+
+      it "returns name from class" do
+        template_name = described_class.call(InvoiceMailer.name, namespace)
+        expect(template_name).to eq("invoice_mailer")
+      end
+
+      it "returns name from modulized class" do
+        template_name = described_class.call(Users::Welcome.name, namespace)
+        expect(template_name).to eq("users/welcome")
+      end
+
+      it "returns blank string from blank name" do
+        template_name = described_class.call("", namespace)
+        expect(template_name).to eq("")
+      end
+
+      it "raises error with nil name" do
+        expect { described_class.call(nil, namespace) }.to raise_error(NoMethodError)
+      end
+    end
+
+    context "with application namespace" do
+      let(:namespace) { Web::Mailers }
+
+      it "returns name from class name" do
+        template_name = described_class.call("SignupMailer", namespace)
+        expect(template_name).to eq("signup_mailer")
+      end
+
+      it "returns name from modulized class name" do
+        template_name = described_class.call(Web::Mailers::SignupMailer.name, namespace)
+        expect(template_name).to eq("signup_mailer")
+      end
+    end
+  end
+end
diff --git a/spec/unit/hanami/mailer/template_spec.rb b/spec/unit/hanami/mailer/template_spec.rb
new file mode 100644
index 0000000..67a011b
--- /dev/null
+++ b/spec/unit/hanami/mailer/template_spec.rb
@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+RSpec.describe Hanami::Mailer::Template do
+  subject    { described_class.new(file) }
+  let(:file) { "spec/support/fixtures/templates/welcome_mailer.txt.erb" }
+
+  describe "#initialize" do
+    context "with existing file" do
+      it "instantiates template" do
+        expect(subject).to be_kind_of(described_class)
+      end
+
+      it "initialize frozen instance" do
+        expect(subject).to be_frozen
+      end
+    end
+
+    context "with missing template engine" do
+      it "returns error" do
+        expect { described_class.new("Gemfile") }.to raise_error(RuntimeError, "No template engine registered for Gemfile")
+      end
+    end
+
+    context "with unexisting file" do
+      it "returns error" do
+        expect { described_class.new("foo.erb") }.to raise_error(Errno::ENOENT)
+      end
+    end
+  end
+
+  describe "#render" do
+    it "renders template" do
+      scope  = Object.new
+      actual = subject.render(scope, greeting: "Hello")
+
+      expect(actual).to eq("This is a txt template\nHello")
+    end
+
+    it "renders with unfrozen object" do
+      scope = Object.new
+      expect(scope).to receive(:dup)
+
+      subject.render(scope, greeting: "Hello")
+    end
+  end
+end
diff --git a/spec/unit/hanami/mailer/templates_finder_spec.rb b/spec/unit/hanami/mailer/templates_finder_spec.rb
new file mode 100644
index 0000000..c4a1230
--- /dev/null
+++ b/spec/unit/hanami/mailer/templates_finder_spec.rb
@@ -0,0 +1,75 @@
+# frozen_string_literal: true
+
+RSpec.describe Hanami::Mailer::TemplatesFinder do
+  subject { described_class.new(root) }
+  # NOTE: please do not change this name, because `#find` specs are relying on
+  # template fixtures. See all the fixtures under: spec/support/fixtures/templates
+  let(:template_name) { "welcome_mailer" }
+  let(:root)          { configuration.root }
+
+  describe "#initialize" do
+    context "with valid root" do
+      it "instantiates a new finder instance" do
+        expect(subject).to be_kind_of(described_class)
+      end
+
+      it "returns a frozen object" do
+        expect(subject).to be_frozen
+      end
+    end
+
+    context "with unexisting root" do
+      let(:root) { "path/to/unexisting" }
+
+      it "raises error" do
+        expect { subject }.to raise_error(Errno::ENOENT)
+      end
+    end
+
+    context "with nil root" do
+      let(:root) { nil }
+
+      it "raises error" do
+        expect { subject }.to raise_error(TypeError)
+      end
+    end
+  end
+
+  describe "#find" do
+    context "with valid template name" do
+      it "returns templates" do
+        actual = subject.find(template_name)
+
+        # It excludes all the files that aren't matching the convention:
+        #
+        #   `<template_name>.<format>.<template_engine>`
+        #
+        # Under `spec/support/fixtures/templates` we have the following files:
+        #
+        #   * welcome_mailer
+        #   * welcome_mailer.erb
+        #   * welcome_mailer.html
+        #   * welcome_mailer.html.erb
+        #   * welcome_mailer.txt.erb
+        #
+        # Only the last two are matching the pattern, here's why we have only
+        # two templates loaded.
+        expect(actual.keys).to eq(%i[html txt])
+        actual.each_value do |template|
+          expect(template).to be_kind_of(Hanami::Mailer::Template)
+          expect(template.instance_variable_get(:@_template).__send__(:file)).to match(%r{spec/support/fixtures/templates/welcome_mailer.(html|txt).erb})
+        end
+      end
+    end
+
+    context "with missing template" do
+      let(:template_name) { "missing_template" }
+
+      it "doesn't return templates" do
+        actual = subject.find(template_name)
+
+        expect(actual).to be_empty
+      end
+    end
+  end
+end
diff --git a/spec/unit/hanami/mailer/unknown_mailer_error_spec.rb b/spec/unit/hanami/mailer/unknown_mailer_error_spec.rb
new file mode 100644
index 0000000..f1f913a
--- /dev/null
+++ b/spec/unit/hanami/mailer/unknown_mailer_error_spec.rb
@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+RSpec.describe Hanami::Mailer::UnknownMailerError do
+  it "inherits from Hanami::Error" do
+    expect(described_class.ancestors).to include(Hanami::Mailer::Error)
+  end
+
+  it "has a custom error message" do
+    mailer = InvoiceMailer
+    expect { raise described_class.new(mailer) }.to raise_error(described_class, "Unknown mailer: #{mailer}. Please finalize the configuration before to use it.")
+  end
+
+  it "has explicit handling for nil" do
+    mailer = nil
+    expect { raise described_class.new(mailer) }.to raise_error(described_class, "Unknown mailer: #{mailer.inspect}. Please finalize the configuration before to use it.")
+  end
+end
diff --git a/spec/unit/hanami/mailer/version_spec.rb b/spec/unit/hanami/mailer/version_spec.rb
index 636b839..d574f32 100644
--- a/spec/unit/hanami/mailer/version_spec.rb
+++ b/spec/unit/hanami/mailer/version_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 RSpec.describe "Hanami::Mailer::VERSION" do
   it "returns current version" do
     expect(Hanami::Mailer::VERSION).to eq("1.1.0")
diff --git a/spec/unit/hanami/mailer_spec.rb b/spec/unit/hanami/mailer_spec.rb
new file mode 100644
index 0000000..5fa7a25
--- /dev/null
+++ b/spec/unit/hanami/mailer_spec.rb
@@ -0,0 +1,168 @@
+# frozen_string_literal: true
+
+RSpec.describe Hanami::Mailer do
+  context "constants" do
+    it "marks them as private" do
+      expect { described_class::CONTENT_TYPES }.to raise_error(NameError)
+    end
+  end
+
+  context ".finalize" do
+    let(:configuration) { Hanami::Mailer::Configuration.new }
+
+    it "finalizes the given configuration" do
+      actual = described_class.finalize(configuration)
+      expect(actual).to be_frozen
+    end
+  end
+
+  context "#initialize" do
+    let(:configuration) { Hanami::Mailer::Configuration.new }
+
+    it "builds an frozen instance" do
+      mailer = described_class.new(configuration: configuration)
+      expect(mailer).to be_frozen
+    end
+
+    it "prevents memoization mutations" do
+      mailer = Class.new(described_class) do
+        def self.name
+          "memoization_attempt"
+        end
+
+        def foo
+          @foo ||= "foo"
+        end
+      end.new(configuration: configuration)
+
+      expect { mailer.foo }.to raise_error(RuntimeError)
+    end
+
+    it "prevents accidental configuration removal" do
+      mailer = Class.new(described_class) do
+        def self.name
+          "configuration_removal"
+        end
+
+        def foo
+          @configuration = nil
+        end
+      end.new(configuration: configuration)
+
+      expect { mailer.foo }.to raise_error(RuntimeError)
+    end
+  end
+
+  context "#deliver" do
+    context "when mailer has from/to defined with DSL" do
+      let(:mailer) { CharsetMailer.new(configuration: configuration) }
+
+      it "delivers email with valid locals" do
+        mail = mailer.deliver({})
+        expect(mail).to be_kind_of(Mail::Message)
+      end
+
+      it "is aliased as #call" do
+        mail = mailer.call({})
+        expect(mail).to be_kind_of(Mail::Message)
+      end
+
+      it "raises error when locals are nil" do
+        expect { mailer.deliver(nil) }.to raise_error(NoMethodError)
+      end
+    end
+
+    context "when from/to are missing" do
+      let(:mailer) { InvoiceMailer.new(configuration: configuration) }
+
+      it "raises error" do
+        expect { mailer.deliver({}) }.to raise_error(Hanami::Mailer::MissingDeliveryDataError)
+      end
+    end
+
+    context "when using #{described_class} directly" do
+      let(:mailer) do
+        described_class.new(configuration: configuration)
+      end
+
+      it "raises error" do
+        expect { mailer.deliver({}) }.to raise_error(NoMethodError)
+      end
+    end
+
+    context "with non-finalized configuration" do
+      let(:configuration) { Hanami::Mailer::Configuration.new }
+
+      let(:mailer) do
+        Class.new(described_class) do
+          def self.name
+            "anonymous_mailer"
+          end
+        end.new(configuration: configuration)
+      end
+
+      it "raises error" do
+        expect { mailer.deliver({}) }.to raise_error(Hanami::Mailer::UnknownMailerError)
+      end
+    end
+
+    context "locals" do
+      let(:mailer) { EventMailer.new(configuration: configuration) }
+      let(:user)   { OpenStruct.new(name: "Luca", email: "luca@domain.test") }
+      let(:event)  { OpenStruct.new(id: 23, title: "Event #23") }
+
+      it "uses locals during the delivery process" do
+        mail = mailer.deliver(user: user, event: event)
+
+        expect(mail.to).to                      eq(["luca@domain.test"])
+        expect(mail.subject).to                 eq("Invitation: Event #23")
+        expect(mail.attachments[0].filename).to eq("invitation-23.ics")
+      end
+    end
+  end
+
+  describe '#render' do
+    describe 'when template is explicitly declared' do
+      let(:mailer) { InvoiceMailer.new(configuration: configuration) }
+
+      it 'renders the given template' do
+        expect(mailer.render(:html, {})).to include(%(<h1>Invoice template</h1>))
+      end
+    end
+
+    describe 'when template is implicitly declared' do
+      let(:mailer) { LazyMailer.new(configuration: configuration) }
+
+      it 'looks for template with same name with inflected classname and render it' do
+        expect(mailer.render(:html, {})).to include(%(Hello World))
+        expect(mailer.render(:txt, {})).to include(%(This is a txt template))
+      end
+    end
+
+    describe 'when mailer defines context' do
+      let(:mailer) { WelcomeMailer.new(configuration: configuration) }
+
+      it 'renders template with defined context' do
+        expect(mailer.render(:txt, {})).to include(%(Ahoy))
+      end
+    end
+
+    describe 'when locals are parsed in' do
+      let(:mailer) { RenderMailer.new(configuration: configuration) }
+      let(:locals) { { user: User.new('Luca') } }
+
+      it 'renders template with parsed locals' do
+        expect(mailer.render(:html, locals)).to include(locals.fetch(:user).name)
+      end
+    end
+
+    describe 'with HAML template engine' do
+      let(:mailer) { TemplateEngineMailer.new(configuration: configuration) }
+      let(:locals) { { user: User.new('MG') } }
+
+      it 'renders template with parsed locals' do
+        expect(mailer.render(:html, locals)).to include(%(<h1>\n#{locals.fetch(:user).name}\n</h1>\n))
+      end
+    end
+  end
+end