diff --git a/actionmailer/lib/action_mailer.rb b/actionmailer/lib/action_mailer.rb
index 55c017e338..668bc99435 100644
--- a/actionmailer/lib/action_mailer.rb
+++ b/actionmailer/lib/action_mailer.rb
@@ -21,15 +21,15 @@
 # WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 #++
 
-require 'abstract_controller'
-require 'action_mailer/version'
+require "abstract_controller"
+require "action_mailer/version"
 
 # Common Active Support usage in Action Mailer
-require 'active_support/rails'
-require 'active_support/core_ext/class'
-require 'active_support/core_ext/module/attr_internal'
-require 'active_support/core_ext/string/inflections'
-require 'active_support/lazy_load_hooks'
+require "active_support/rails"
+require "active_support/core_ext/class"
+require "active_support/core_ext/module/attr_internal"
+require "active_support/core_ext/string/inflections"
+require "active_support/lazy_load_hooks"
 
 module ActionMailer
   extend ::ActiveSupport::Autoload
@@ -43,14 +43,14 @@ module ActionMailer
   autoload :InlinePreviewInterceptor
   autoload :MailHelper
   autoload :Preview
-  autoload :Previews, 'action_mailer/preview'
+  autoload :Previews, "action_mailer/preview"
   autoload :TestCase
   autoload :TestHelper
   autoload :MessageDelivery
   autoload :DeliveryJob
 end
 
-autoload :Mime, 'action_dispatch/http/mime_type'
+autoload :Mime, "action_dispatch/http/mime_type"
 
 ActiveSupport.on_load(:action_view) do
   ActionView::Base.default_formats ||= Mime::SET.symbols
diff --git a/actionmailer/lib/action_mailer/base.rb b/actionmailer/lib/action_mailer/base.rb
index 0f7c3b54e9..315321620b 100644
--- a/actionmailer/lib/action_mailer/base.rb
+++ b/actionmailer/lib/action_mailer/base.rb
@@ -1,11 +1,11 @@
-require 'mail'
-require 'action_mailer/collector'
-require 'active_support/core_ext/string/inflections'
-require 'active_support/core_ext/hash/except'
-require 'active_support/core_ext/module/anonymous'
+require "mail"
+require "action_mailer/collector"
+require "active_support/core_ext/string/inflections"
+require "active_support/core_ext/hash/except"
+require "active_support/core_ext/module/anonymous"
 
-require 'action_mailer/log_subscriber'
-require 'action_mailer/rescuable'
+require "action_mailer/log_subscriber"
+require "action_mailer/rescuable"
 
 module ActionMailer
   # Action Mailer allows you to send email from your application using a mailer model and views.
@@ -598,7 +598,7 @@ module ActionMailer
     end
 
     class NullMail #:nodoc:
-      def body; '' end
+      def body; "" end
       def header; {} end
 
       def respond_to?(string, include_all=false)
@@ -864,7 +864,7 @@ module ActionMailer
     # humanized version of the <tt>action_name</tt>.
     # If the subject has interpolations, you can pass them through the +interpolations+ parameter.
     def default_i18n_subject(interpolations = {})
-      mailer_scope = self.class.mailer_name.tr('/', '.')
+      mailer_scope = self.class.mailer_name.tr("/", ".")
       I18n.t(:subject, interpolations.merge(scope: [mailer_scope, action_name], default: action_name.humanize))
     end
 
@@ -925,7 +925,7 @@ module ActionMailer
     def each_template(paths, name, &block)
       templates = lookup_context.find_all(name, paths)
       if templates.empty?
-        raise ActionView::MissingTemplate.new(paths, name, paths, false, 'mailer')
+        raise ActionView::MissingTemplate.new(paths, name, paths, false, "mailer")
       else
         templates.uniq(&:formats).each(&block)
       end
diff --git a/actionmailer/lib/action_mailer/collector.rb b/actionmailer/lib/action_mailer/collector.rb
index e8883a8235..d97a73d65a 100644
--- a/actionmailer/lib/action_mailer/collector.rb
+++ b/actionmailer/lib/action_mailer/collector.rb
@@ -1,6 +1,6 @@
-require 'abstract_controller/collector'
-require 'active_support/core_ext/hash/reverse_merge'
-require 'active_support/core_ext/array/extract_options'
+require "abstract_controller/collector"
+require "active_support/core_ext/hash/reverse_merge"
+require "active_support/core_ext/array/extract_options"
 
 module ActionMailer
   class Collector
diff --git a/actionmailer/lib/action_mailer/delivery_job.rb b/actionmailer/lib/action_mailer/delivery_job.rb
index d371c1b61a..a617daa87e 100644
--- a/actionmailer/lib/action_mailer/delivery_job.rb
+++ b/actionmailer/lib/action_mailer/delivery_job.rb
@@ -1,4 +1,4 @@
-require 'active_job'
+require "active_job"
 
 module ActionMailer
   # The <tt>ActionMailer::DeliveryJob</tt> class is used when you
diff --git a/actionmailer/lib/action_mailer/delivery_methods.rb b/actionmailer/lib/action_mailer/delivery_methods.rb
index c78ff555f3..6be2c91da6 100644
--- a/actionmailer/lib/action_mailer/delivery_methods.rb
+++ b/actionmailer/lib/action_mailer/delivery_methods.rb
@@ -1,4 +1,4 @@
-require 'tmpdir'
+require "tmpdir"
 
 module ActionMailer
   # This module handles everything related to mail delivery, from registering
@@ -25,7 +25,7 @@ module ActionMailer
       add_delivery_method :smtp, Mail::SMTP,
         address:              "localhost",
         port:                 25,
-        domain:               'localhost.localdomain',
+        domain:               "localhost.localdomain",
         user_name:            nil,
         password:             nil,
         authentication:       nil,
@@ -35,8 +35,8 @@ module ActionMailer
         location: defined?(Rails.root) ? "#{Rails.root}/tmp/mails" : "#{Dir.tmpdir}/mails"
 
       add_delivery_method :sendmail, Mail::Sendmail,
-        location:  '/usr/sbin/sendmail',
-        arguments: '-i'
+        location:  "/usr/sbin/sendmail",
+        arguments: "-i"
 
       add_delivery_method :test, Mail::TestMailer
     end
diff --git a/actionmailer/lib/action_mailer/inline_preview_interceptor.rb b/actionmailer/lib/action_mailer/inline_preview_interceptor.rb
index 419d6c7b93..fd9a7bce15 100644
--- a/actionmailer/lib/action_mailer/inline_preview_interceptor.rb
+++ b/actionmailer/lib/action_mailer/inline_preview_interceptor.rb
@@ -1,4 +1,4 @@
-require 'base64'
+require "base64"
 
 module ActionMailer
   # Implements a mailer preview interceptor that converts image tag src attributes
diff --git a/actionmailer/lib/action_mailer/log_subscriber.rb b/actionmailer/lib/action_mailer/log_subscriber.rb
index 2867bf90fb..2c058ccf66 100644
--- a/actionmailer/lib/action_mailer/log_subscriber.rb
+++ b/actionmailer/lib/action_mailer/log_subscriber.rb
@@ -1,4 +1,4 @@
-require 'active_support/log_subscriber'
+require "active_support/log_subscriber"
 
 module ActionMailer
   # Implements the ActiveSupport::LogSubscriber for logging notifications when
@@ -7,7 +7,7 @@ module ActionMailer
     # An email was delivered.
     def deliver(event)
       info do
-        recipients = Array(event.payload[:to]).join(', ')
+        recipients = Array(event.payload[:to]).join(", ")
         "Sent mail to #{recipients} (#{event.duration.round(1)}ms)"
       end
 
diff --git a/actionmailer/lib/action_mailer/mail_helper.rb b/actionmailer/lib/action_mailer/mail_helper.rb
index 239974e7b1..e04fc08866 100644
--- a/actionmailer/lib/action_mailer/mail_helper.rb
+++ b/actionmailer/lib/action_mailer/mail_helper.rb
@@ -54,7 +54,7 @@ module ActionMailer
       sentences = [[]]
 
       text.split.each do |word|
-        if sentences.first.present? && (sentences.last + [word]).join(' ').length > len
+        if sentences.first.present? && (sentences.last + [word]).join(" ").length > len
           sentences << [word]
         else
           sentences.last << word
diff --git a/actionmailer/lib/action_mailer/message_delivery.rb b/actionmailer/lib/action_mailer/message_delivery.rb
index c5ba5f9f1d..994d297768 100644
--- a/actionmailer/lib/action_mailer/message_delivery.rb
+++ b/actionmailer/lib/action_mailer/message_delivery.rb
@@ -1,4 +1,4 @@
-require 'delegate'
+require "delegate"
 
 module ActionMailer
   # The <tt>ActionMailer::MessageDelivery</tt> class is used by
diff --git a/actionmailer/lib/action_mailer/preview.rb b/actionmailer/lib/action_mailer/preview.rb
index aab92fe8db..de9f945c36 100644
--- a/actionmailer/lib/action_mailer/preview.rb
+++ b/actionmailer/lib/action_mailer/preview.rb
@@ -1,4 +1,4 @@
-require 'active_support/descendants_tracker'
+require "active_support/descendants_tracker"
 
 module ActionMailer
   module Previews #:nodoc:
@@ -90,7 +90,7 @@ module ActionMailer
 
       # Returns the underscored name of the mailer preview without the suffix.
       def preview_name
-        name.sub(/Preview$/, '').underscore
+        name.sub(/Preview$/, "").underscore
       end
 
       protected
diff --git a/actionmailer/lib/action_mailer/railtie.rb b/actionmailer/lib/action_mailer/railtie.rb
index a727ed38e9..c47d7781cc 100644
--- a/actionmailer/lib/action_mailer/railtie.rb
+++ b/actionmailer/lib/action_mailer/railtie.rb
@@ -1,4 +1,4 @@
-require 'active_job/railtie'
+require "active_job/railtie"
 require "action_mailer"
 require "rails"
 require "abstract_controller/railties/routes_helpers"
@@ -18,7 +18,7 @@ module ActionMailer
 
       if app.config.force_ssl
         options.default_url_options ||= {}
-        options.default_url_options[:protocol] ||= 'https'
+        options.default_url_options[:protocol] ||= "https"
       end
 
       options.assets_dir      ||= paths["public"].first
@@ -61,8 +61,8 @@ module ActionMailer
 
       if options.show_previews
         app.routes.prepend do
-          get '/rails/mailers'         => "rails/mailers#index", internal: true
-          get '/rails/mailers/*path'   => "rails/mailers#preview", internal: true
+          get "/rails/mailers"         => "rails/mailers#index", internal: true
+          get "/rails/mailers/*path"   => "rails/mailers#preview", internal: true
         end
 
         if options.preview_path
diff --git a/actionmailer/lib/action_mailer/test_case.rb b/actionmailer/lib/action_mailer/test_case.rb
index b045e883ad..c224fa11d4 100644
--- a/actionmailer/lib/action_mailer/test_case.rb
+++ b/actionmailer/lib/action_mailer/test_case.rb
@@ -1,5 +1,5 @@
-require 'active_support/test_case'
-require 'rails-dom-testing'
+require "active_support/test_case"
+require "rails-dom-testing"
 
 module ActionMailer
   class NonInferrableMailerError < ::StandardError
@@ -99,7 +99,7 @@ module ActionMailer
         def set_expected_mail # :nodoc:
           @expected = Mail.new
           @expected.content_type ["text", "plain", { "charset" => charset }]
-          @expected.mime_version = '1.0'
+          @expected.mime_version = "1.0"
         end
 
       private
@@ -113,7 +113,7 @@ module ActionMailer
         end
 
         def read_fixture(action)
-          IO.readlines(File.join(Rails.root, 'test', 'fixtures', self.class.mailer_class.name.underscore, action))
+          IO.readlines(File.join(Rails.root, "test", "fixtures", self.class.mailer_class.name.underscore, action))
         end
     end
 
diff --git a/actionmailer/lib/action_mailer/test_helper.rb b/actionmailer/lib/action_mailer/test_helper.rb
index e423aac389..c17ecad4c6 100644
--- a/actionmailer/lib/action_mailer/test_helper.rb
+++ b/actionmailer/lib/action_mailer/test_helper.rb
@@ -1,4 +1,4 @@
-require 'active_job'
+require "active_job"
 
 module ActionMailer
   # Provides helper methods for testing Action Mailer, including #assert_emails
diff --git a/actionmailer/lib/action_mailer/version.rb b/actionmailer/lib/action_mailer/version.rb
index 06f80a8fdc..8452d6370e 100644
--- a/actionmailer/lib/action_mailer/version.rb
+++ b/actionmailer/lib/action_mailer/version.rb
@@ -1,4 +1,4 @@
-require_relative 'gem_version'
+require_relative "gem_version"
 
 module ActionMailer
   # Returns the version of the currently loaded Action Mailer as a
diff --git a/actionmailer/lib/rails/generators/mailer/mailer_generator.rb b/actionmailer/lib/rails/generators/mailer/mailer_generator.rb
index 01bdfb0685..4b74fb4258 100644
--- a/actionmailer/lib/rails/generators/mailer/mailer_generator.rb
+++ b/actionmailer/lib/rails/generators/mailer/mailer_generator.rb
@@ -8,11 +8,11 @@ module Rails
       check_class_collision suffix: "Mailer"
 
       def create_mailer_file
-        template "mailer.rb", File.join('app/mailers', class_path, "#{file_name}_mailer.rb")
+        template "mailer.rb", File.join("app/mailers", class_path, "#{file_name}_mailer.rb")
 
         in_root do
           if self.behavior == :invoke && !File.exist?(application_mailer_file_name)
-            template 'application_mailer.rb', application_mailer_file_name
+            template "application_mailer.rb", application_mailer_file_name
           end
         end
       end
@@ -21,7 +21,7 @@ module Rails
 
       protected
         def file_name
-          @_file_name ||= super.gsub(/_mailer/i, '')
+          @_file_name ||= super.gsub(/_mailer/i, "")
         end
 
       private