diff --git a/Gemfile b/Gemfile
index 65f1a6f190..bb5cba25a7 100644
--- a/Gemfile
+++ b/Gemfile
@@ -4,3 +4,5 @@ git_source(:github) { |repo_path| "https://github.com/#{repo_path}.git" }
 gemspec
 
 gem "rails", github: "rails/rails"
+
+gem "aws-sdk-sns"
diff --git a/Gemfile.lock b/Gemfile.lock
index d01cb7bd76..402a6e8a10 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -66,20 +66,46 @@ PATH
   remote: .
   specs:
     actionmailbox (0.1.0)
+      http (>= 4.0.0)
       rails (>= 5.2.0)
 
 GEM
   remote: https://rubygems.org/
   specs:
+    addressable (2.5.2)
+      public_suffix (>= 2.0.2, < 4.0)
+    aws-eventstream (1.0.1)
+    aws-partitions (1.105.0)
+    aws-sdk-core (3.30.0)
+      aws-eventstream (~> 1.0)
+      aws-partitions (~> 1.0)
+      aws-sigv4 (~> 1.0)
+      jmespath (~> 1.0)
+    aws-sdk-sns (1.5.0)
+      aws-sdk-core (~> 3, >= 3.26.0)
+      aws-sigv4 (~> 1.0)
+    aws-sigv4 (1.0.3)
     builder (3.2.3)
     byebug (10.0.2)
     concurrent-ruby (1.0.5)
     crass (1.0.4)
+    domain_name (0.5.20180417)
+      unf (>= 0.0.5, < 1.0.0)
     erubi (1.7.1)
     globalid (0.4.1)
       activesupport (>= 4.2.0)
+    http (4.0.0)
+      addressable (~> 2.3)
+      http-cookie (~> 1.0)
+      http-form_data (~> 2.0)
+      http_parser.rb (~> 0.6.0)
+    http-cookie (1.0.3)
+      domain_name (~> 0.5)
+    http-form_data (2.1.1)
+    http_parser.rb (0.6.0)
     i18n (1.1.0)
       concurrent-ruby (~> 1.0)
+    jmespath (1.4.0)
     loofah (2.2.2)
       crass (~> 1.0.2)
       nokogiri (>= 1.5.9)
@@ -95,6 +121,7 @@ GEM
     nio4r (2.3.1)
     nokogiri (1.8.5)
       mini_portile2 (~> 2.3.0)
+    public_suffix (3.0.3)
     rack (2.0.5)
     rack-test (1.1.0)
       rack (>= 1.0, < 3)
@@ -116,6 +143,9 @@ GEM
     thread_safe (0.3.6)
     tzinfo (1.2.5)
       thread_safe (~> 0.1)
+    unf (0.1.4)
+      unf_ext
+    unf_ext (0.0.7.5)
     websocket-driver (0.7.0)
       websocket-extensions (>= 0.1.0)
     websocket-extensions (0.1.3)
@@ -125,6 +155,7 @@ PLATFORMS
 
 DEPENDENCIES
   actionmailbox!
+  aws-sdk-sns
   bundler (~> 1.15)
   byebug
   rails!
diff --git a/actionmailbox.gemspec b/actionmailbox.gemspec
index e413ac57f6..e3890ab574 100644
--- a/actionmailbox.gemspec
+++ b/actionmailbox.gemspec
@@ -16,6 +16,7 @@ Gem::Specification.new do |s|
   s.required_ruby_version = ">= 2.5.0"
 
   s.add_dependency "rails", ">= 5.2.0"
+  s.add_dependency "http", ">= 4.0.0"
 
   s.add_development_dependency "bundler", "~> 1.15"
   s.add_development_dependency "sqlite3"
diff --git a/app/controllers/action_mailbox/base_controller.rb b/app/controllers/action_mailbox/base_controller.rb
new file mode 100644
index 0000000000..c234ecd250
--- /dev/null
+++ b/app/controllers/action_mailbox/base_controller.rb
@@ -0,0 +1,42 @@
+class ActionMailbox::BaseController < ActionController::Base
+  skip_forgery_protection
+
+  def self.prepare
+    # Override in concrete controllers to run code on load.
+  end
+
+  before_action :ensure_configured
+
+  private
+    def ensure_configured
+      unless ActionMailbox.ingress == ingress_name
+        head :not_found
+      end
+    end
+
+    def ingress_name
+      self.class.name[/^ActionMailbox::Ingresses::(.*?)::/, 1].underscore.to_sym
+    end
+
+
+    def authenticate_by_password
+      if password.present?
+        http_basic_authenticate_or_request_with username: "actionmailbox", password: password, realm: "Action Mailbox"
+      else
+        raise ArgumentError, "Missing required ingress credentials"
+      end
+    end
+
+    def password
+      Rails.application.credentials.dig(:action_mailbox, :ingress_password) || ENV["RAILS_INBOUND_EMAIL_PASSWORD"]
+    end
+
+
+    # TODO: Extract to ActionController::HttpAuthentication
+    def http_basic_authenticate_or_request_with(username:, password:, realm: nil)
+      authenticate_or_request_with_http_basic(realm || "Application") do |given_username, given_password|
+        ActiveSupport::SecurityUtils.secure_compare(given_username, username) &
+          ActiveSupport::SecurityUtils.secure_compare(given_password, password)
+      end
+    end
+end
diff --git a/app/controllers/action_mailbox/inbound_emails_controller.rb b/app/controllers/action_mailbox/inbound_emails_controller.rb
deleted file mode 100644
index ec9bd6f229..0000000000
--- a/app/controllers/action_mailbox/inbound_emails_controller.rb
+++ /dev/null
@@ -1,17 +0,0 @@
-# TODO: Add access protection using basic auth with verified tokens. Maybe coming from credentials by default?
-# TODO: Spam/malware catching?
-# TODO: Specific bounces for SMTP good citizenship: 200/404/400
-class ActionMailbox::InboundEmailsController < ActionController::Base
-  skip_forgery_protection
-  before_action :require_rfc822_message, only: :create
-
-  def create
-    ActionMailbox::InboundEmail.create_and_extract_message_id!(params[:message])
-    head :created
-  end
-
-  private
-    def require_rfc822_message
-      head :unsupported_media_type unless params.require(:message).content_type == 'message/rfc822'
-    end
-end
diff --git a/app/controllers/action_mailbox/ingresses/amazon/inbound_emails_controller.rb b/app/controllers/action_mailbox/ingresses/amazon/inbound_emails_controller.rb
new file mode 100644
index 0000000000..d3998be2d4
--- /dev/null
+++ b/app/controllers/action_mailbox/ingresses/amazon/inbound_emails_controller.rb
@@ -0,0 +1,21 @@
+class ActionMailbox::Ingresses::Amazon::InboundEmailsController < ActionMailbox::BaseController
+  before_action :authenticate
+
+  cattr_accessor :verifier
+
+  def self.prepare
+    self.verifier ||= begin
+      require "aws-sdk-sns/message_verifier"
+      Aws::SNS::MessageVerifier.new
+    end
+  end
+
+  def create
+    ActionMailbox::InboundEmail.create_and_extract_message_id! params.require(:content)
+  end
+
+  private
+    def authenticate
+      head :unauthorized unless verifier.authentic?(request.body)
+    end
+end
diff --git a/app/controllers/action_mailbox/ingresses/mailgun/inbound_emails_controller.rb b/app/controllers/action_mailbox/ingresses/mailgun/inbound_emails_controller.rb
new file mode 100644
index 0000000000..e878192603
--- /dev/null
+++ b/app/controllers/action_mailbox/ingresses/mailgun/inbound_emails_controller.rb
@@ -0,0 +1,58 @@
+class ActionMailbox::Ingresses::Mailgun::InboundEmailsController < ActionMailbox::BaseController
+  before_action :authenticate
+
+  def create
+    ActionMailbox::InboundEmail.create_and_extract_message_id! params.require("body-mime")
+  end
+
+  private
+    def authenticate
+      head :unauthorized unless authenticated?
+    end
+
+    def authenticated?
+      if key.present?
+        Authenticator.new(
+          key:       key,
+          timestamp: params.require(:timestamp),
+          token:     params.require(:token),
+          signature: params.require(:signature)
+        ).authenticated?
+      else
+        raise ArgumentError, <<~MESSAGE.squish
+          Missing required Mailgun API key. Set action_mailbox.mailgun_api_key in your application's
+          encrypted credentials or provide the MAILGUN_INGRESS_API_KEY environment variable.
+        MESSAGE
+      end
+    end
+
+    def key
+      Rails.application.credentials.dig(:action_mailbox, :mailgun_api_key) || ENV["MAILGUN_INGRESS_API_KEY"]
+    end
+
+    class Authenticator
+      attr_reader :key, :timestamp, :token, :signature
+
+      def initialize(key:, timestamp:, token:, signature:)
+        @key, @timestamp, @token, @signature = key, Integer(timestamp), token, signature
+      end
+
+      def authenticated?
+        signed? && recent?
+      end
+
+      private
+        def signed?
+          ActiveSupport::SecurityUtils.secure_compare signature, expected_signature
+        end
+
+        # Allow for 2 minutes of drift between Mailgun time and local server time.
+        def recent?
+          Time.at(timestamp) >= 2.minutes.ago
+        end
+
+        def expected_signature
+          OpenSSL::HMAC.hexdigest OpenSSL::Digest::SHA256.new, key, "#{timestamp}#{token}"
+        end
+    end
+end
diff --git a/app/controllers/action_mailbox/ingresses/mandrill/inbound_emails_controller.rb b/app/controllers/action_mailbox/ingresses/mandrill/inbound_emails_controller.rb
new file mode 100644
index 0000000000..b32b254076
--- /dev/null
+++ b/app/controllers/action_mailbox/ingresses/mandrill/inbound_emails_controller.rb
@@ -0,0 +1,65 @@
+class ActionMailbox::Ingresses::Mandrill::InboundEmailsController < ActionMailbox::BaseController
+  before_action :authenticate
+
+  def create
+    raw_emails.each { |raw_email| ActionMailbox::InboundEmail.create_and_extract_message_id! raw_email }
+    head :ok
+  rescue JSON::ParserError => error
+    logger.error error.message
+    head :unprocessable_entity
+  end
+
+  private
+    def raw_emails
+      events.select { |event| event["event"] == "inbound" }.collect { |event| event.dig("msg", "raw_msg") }
+    end
+
+    def events
+      JSON.parse params.require(:mandrill_events)
+    end
+
+
+    def authenticate
+      head :unauthorized unless authenticated?
+    end
+
+    def authenticated?
+      if key.present?
+        Authenticator.new(request, key).authenticated?
+      else
+        raise ArgumentError, <<~MESSAGE.squish
+          Missing required Mandrill API key. Set action_mailbox.mandrill_api_key in your application's
+          encrypted credentials or provide the MANDRILL_INGRESS_API_KEY environment variable.
+        MESSAGE
+      end
+    end
+
+    def key
+      Rails.application.credentials.dig(:action_mailbox, :mandrill_api_key) || ENV["MANDRILL_INGRESS_API_KEY"]
+    end
+
+    class Authenticator
+      attr_reader :request, :key
+
+      def initialize(request, key)
+        @request, @key = request, key
+      end
+
+      def authenticated?
+        ActiveSupport::SecurityUtils.secure_compare given_signature, expected_signature
+      end
+
+      private
+        def given_signature
+          request.headers["X-Mandrill-Signature"]
+        end
+
+        def expected_signature
+          Base64.strict_encode64 OpenSSL::HMAC.digest(OpenSSL::Digest::SHA1.new, key, message)
+        end
+
+        def message
+          request.url + request.POST.sort.flatten.join
+        end
+    end
+end
diff --git a/app/controllers/action_mailbox/ingresses/postfix/inbound_emails_controller.rb b/app/controllers/action_mailbox/ingresses/postfix/inbound_emails_controller.rb
new file mode 100644
index 0000000000..133accf651
--- /dev/null
+++ b/app/controllers/action_mailbox/ingresses/postfix/inbound_emails_controller.rb
@@ -0,0 +1,14 @@
+class ActionMailbox::Ingresses::Postfix::InboundEmailsController < ActionMailbox::BaseController
+  before_action :authenticate_by_password, :require_valid_rfc822_message
+
+  def create
+    ActionMailbox::InboundEmail.create_and_extract_message_id! request.body.read
+  end
+
+  private
+    def require_valid_rfc822_message
+      unless request.content_type == "message/rfc822"
+        head :unsupported_media_type
+      end
+    end
+end
diff --git a/app/controllers/action_mailbox/ingresses/sendgrid/inbound_emails_controller.rb b/app/controllers/action_mailbox/ingresses/sendgrid/inbound_emails_controller.rb
new file mode 100644
index 0000000000..b856eb5b94
--- /dev/null
+++ b/app/controllers/action_mailbox/ingresses/sendgrid/inbound_emails_controller.rb
@@ -0,0 +1,7 @@
+class ActionMailbox::Ingresses::Sendgrid::InboundEmailsController < ActionMailbox::BaseController
+  before_action :authenticate_by_password
+
+  def create
+    ActionMailbox::InboundEmail.create_and_extract_message_id! params.require(:email)
+  end
+end
diff --git a/app/models/action_mailbox/inbound_email/message_id.rb b/app/models/action_mailbox/inbound_email/message_id.rb
index a1ec5c0437..5cfcadaba1 100644
--- a/app/models/action_mailbox/inbound_email/message_id.rb
+++ b/app/models/action_mailbox/inbound_email/message_id.rb
@@ -6,14 +6,16 @@ module ActionMailbox::InboundEmail::MessageId
   end
 
   module ClassMethods
-    def create_and_extract_message_id!(raw_email, **options)
-      create! raw_email: raw_email, message_id: extract_message_id(raw_email), **options
+    def create_and_extract_message_id!(source, **options)
+      create! message_id: extract_message_id(source), **options do |inbound_email|
+        inbound_email.raw_email.attach io: StringIO.new(source), filename: "message.eml", content_type: "message/rfc822"
+      end
     end
 
     private
-      def extract_message_id(raw_email)
-        mail_from_source(raw_email.read).message_id
-      rescue
+      def extract_message_id(source)
+        mail_from_source(source).message_id
+      rescue => e
         # FIXME: Add logging with "Couldn't extract Message ID, so will generating a new random ID instead"
       end
   end
diff --git a/config/routes.rb b/config/routes.rb
index 733f137262..99a15d1d32 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -1,11 +1,20 @@
 # frozen_string_literal: true
 
 Rails.application.routes.draw do
-  post "/rails/action_mailbox/inbound_emails" => "action_mailbox/inbound_emails#create", as: :rails_inbound_emails
+  scope "/rails/action_mailbox", module: "action_mailbox/ingresses" do
+    post "/amazon/inbound_emails"   => "amazon/inbound_emails#create",   as: :rails_amazon_inbound_emails
+    post "/mandrill/inbound_emails" => "mandrill/inbound_emails#create", as: :rails_mandrill_inbound_emails
+    post "/postfix/inbound_emails"  => "postfix/inbound_emails#create",  as: :rails_postfix_inbound_emails
+    post "/sendgrid/inbound_emails" => "sendgrid/inbound_emails#create", as: :rails_sendgrid_inbound_emails
+
+    # Mailgun requires that a webhook's URL end in 'mime' for it to receive the raw contents of emails.
+    post "/mailgun/inbound_emails/mime" => "mailgun/inbound_emails#create", as: :rails_mailgun_inbound_emails
+  end
 
   # TODO: Should these be mounted within the engine only?
   scope "rails/conductor/action_mailbox/", module: "rails/conductor/action_mailbox" do
-    resources :inbound_emails, as: :rails_conductor_inbound_emails
-    post ":inbound_email_id/reroute" => "reroutes#create", as: :rails_conductor_inbound_email_reroute
+    resources :inbound_emails, as: :rails_conductor_inbound_emails do
+      post "reroute" => "reroutes#create"
+    end
   end
 end
diff --git a/lib/action_mailbox.rb b/lib/action_mailbox.rb
index ae37cb84ed..fbc8122d9d 100644
--- a/lib/action_mailbox.rb
+++ b/lib/action_mailbox.rb
@@ -6,6 +6,7 @@ module ActionMailbox
   autoload :Base
   autoload :Router
 
+  mattr_accessor :ingress
   mattr_accessor :logger
   mattr_accessor :incinerate_after, default: 30.days
 end
diff --git a/lib/action_mailbox/engine.rb b/lib/action_mailbox/engine.rb
index b4758aacb5..cf438d8f24 100644
--- a/lib/action_mailbox/engine.rb
+++ b/lib/action_mailbox/engine.rb
@@ -14,5 +14,17 @@ module ActionMailbox
         ActionMailbox.incinerate_after = app.config.action_mailbox.incinerate_after || 30.days
       end
     end
+
+    initializer "action_mailbox.ingress" do
+      config.after_initialize do |app|
+        if ActionMailbox.ingress = app.config.action_mailbox.ingress.presence
+          config.to_prepare do
+            if ingress_controller_class = "ActionMailbox::Ingresses::#{ActionMailbox.ingress.to_s.classify}::InboundEmailsController".safe_constantize
+              ingress_controller_class.prepare
+            end
+          end
+        end
+      end
+    end
   end
 end
diff --git a/lib/action_mailbox/test_helper.rb b/lib/action_mailbox/test_helper.rb
index a74ea8ef57..23b2bb02ca 100644
--- a/lib/action_mailbox/test_helper.rb
+++ b/lib/action_mailbox/test_helper.rb
@@ -5,18 +5,15 @@ module ActionMailbox
     # Create an InboundEmail record using an eml fixture in the format of message/rfc822
     # referenced with +fixture_name+ located in +test/fixtures/files/fixture_name+.
     def create_inbound_email_from_fixture(fixture_name, status: :processing)
-      create_inbound_email file_fixture(fixture_name), filename: fixture_name, status: status
+      create_inbound_email file_fixture(fixture_name).read, status: status
     end
 
     def create_inbound_email_from_mail(status: :processing, **mail_options)
-      raw_email = Tempfile.new.tap { |io| io.write Mail.new(mail_options).to_s }
-      create_inbound_email(raw_email, status: status)
+      create_inbound_email Mail.new(mail_options).to_s, status: status
     end
 
-    def create_inbound_email(io, filename: 'mail.eml', status: :processing)
-      ActionMailbox::InboundEmail.create_and_extract_message_id! \
-        ActionDispatch::Http::UploadedFile.new(tempfile: io, filename: filename, type: 'message/rfc822'),
-        status: status
+    def create_inbound_email(source, status: :processing)
+      ActionMailbox::InboundEmail.create_and_extract_message_id! source, status: status
     end
 
     def receive_inbound_email_from_fixture(*args)
diff --git a/lib/tasks/ingress.rake b/lib/tasks/ingress.rake
new file mode 100644
index 0000000000..510951aa2a
--- /dev/null
+++ b/lib/tasks/ingress.rake
@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+namespace :action_mailbox do
+  namespace :ingress do
+    desc "Pipe an inbound email from STDIN to the Postfix ingress at the given URL"
+    task :postfix do
+      require "active_support"
+      require "active_support/core_ext/object/blank"
+      require "http"
+
+      unless url = ENV["URL"].presence
+        abort "5.3.5 URL is required"
+      end
+
+      unless password = ENV["INGRESS_PASSWORD"].presence
+        abort "5.3.5 INGRESS_PASSWORD is required"
+      end
+
+      begin
+        response = HTTP.basic_auth(user: "actionmailbox", pass: password)
+          .timeout(connect: 1, write: 10, read: 10)
+          .post(url, headers: { "Content-Type" => "message/rfc822", "User-Agent" => ENV.fetch("USER_AGENT", "Postfix") }, body: STDIN)
+
+        case
+        when response.status.success?
+          puts "2.0.0 HTTP #{response.status}"
+        when response.status.unauthorized?
+          abort "4.7.0 HTTP #{response.status}"
+        when response.status.unsupported_media_type?
+          abort "5.6.1 HTTP #{response.status}"
+        else
+          abort "4.0.0 HTTP #{response.status}"
+        end
+      rescue HTTP::ConnectionError => error
+        abort "4.4.2 Error connecting to the Postfix ingress: #{error.message}"
+      rescue HTTP::TimeoutError
+        abort "4.4.7 Timed out piping to the Postfix ingress"
+      end
+    end
+  end
+end
diff --git a/test/controllers/ingresses/amazon/inbound_emails_controller_test.rb b/test/controllers/ingresses/amazon/inbound_emails_controller_test.rb
new file mode 100644
index 0000000000..c36c500cbe
--- /dev/null
+++ b/test/controllers/ingresses/amazon/inbound_emails_controller_test.rb
@@ -0,0 +1,20 @@
+require "test_helper"
+
+ActionMailbox::Ingresses::Amazon::InboundEmailsController.verifier =
+  Module.new { def self.authentic?(message); true; end }
+
+class ActionMailbox::Ingresses::Amazon::InboundEmailsControllerTest < ActionDispatch::IntegrationTest
+  setup { ActionMailbox.ingress = :amazon }
+
+  test "receiving an inbound email from Amazon" do
+    assert_difference -> { ActionMailbox::InboundEmail.count }, +1 do
+      post rails_amazon_inbound_emails_url, params: { content: file_fixture("../files/welcome.eml").read }, as: :json
+    end
+
+    assert_response :no_content
+
+    inbound_email = ActionMailbox::InboundEmail.last
+    assert_equal file_fixture("../files/welcome.eml").read, inbound_email.raw_email.download
+    assert_equal "0CB459E0-0336-41DA-BC88-E6E28C697DDB@37signals.com", inbound_email.message_id
+  end
+end
diff --git a/test/controllers/ingresses/mailgun/inbound_emails_controller_test.rb b/test/controllers/ingresses/mailgun/inbound_emails_controller_test.rb
new file mode 100644
index 0000000000..c5ec71013e
--- /dev/null
+++ b/test/controllers/ingresses/mailgun/inbound_emails_controller_test.rb
@@ -0,0 +1,89 @@
+require "test_helper"
+
+ENV["MAILGUN_INGRESS_API_KEY"] = "tbsy84uSV1Kt3ZJZELY2TmShPRs91E3yL4tzf96297vBCkDWgL"
+
+class ActionMailbox::Ingresses::Mailgun::InboundEmailsControllerTest < ActionDispatch::IntegrationTest
+  setup { ActionMailbox.ingress = :mailgun }
+
+  test "receiving an inbound email from Mailgun" do
+    assert_difference -> { ActionMailbox::InboundEmail.count }, +1 do
+      travel_to "2018-10-09 15:15:00 EDT"
+      post rails_mailgun_inbound_emails_url, params: {
+        timestamp: 1539112500,
+        token: "7VwW7k6Ak7zcTwoSoNm7aTtbk1g67MKAnsYLfUB7PdszbgR5Xi",
+        signature: "ef24c5225322217bb065b80bb54eb4f9206d764e3e16abab07f0a64d1cf477cc",
+        "body-mime" => file_fixture("../files/welcome.eml").read
+      }
+    end
+
+    assert_response :no_content
+
+    inbound_email = ActionMailbox::InboundEmail.last
+    assert_equal file_fixture("../files/welcome.eml").read, inbound_email.raw_email.download
+    assert_equal "0CB459E0-0336-41DA-BC88-E6E28C697DDB@37signals.com", inbound_email.message_id
+  end
+
+  test "rejecting a delayed inbound email from Mailgun" do
+    assert_no_difference -> { ActionMailbox::InboundEmail.count } do
+      travel_to "2018-10-09 15:26:00 EDT"
+      post rails_mailgun_inbound_emails_url, params: {
+        timestamp: 1539112500,
+        token: "7VwW7k6Ak7zcTwoSoNm7aTtbk1g67MKAnsYLfUB7PdszbgR5Xi",
+        signature: "ef24c5225322217bb065b80bb54eb4f9206d764e3e16abab07f0a64d1cf477cc",
+        "body-mime" => file_fixture("../files/welcome.eml").read
+      }
+    end
+
+    assert_response :unauthorized
+  end
+
+  test "rejecting a forged inbound email from Mailgun" do
+    assert_no_difference -> { ActionMailbox::InboundEmail.count } do
+      travel_to "2018-10-09 15:15:00 EDT"
+      post rails_mailgun_inbound_emails_url, params: {
+        timestamp: 1539112500,
+        token: "Zx8mJBiGmiiyyfWnho3zKyjCg2pxLARoCuBM7X9AKCioShGiMX",
+        signature: "ef24c5225322217bb065b80bb54eb4f9206d764e3e16abab07f0a64d1cf477cc",
+        "body-mime" => file_fixture("../files/welcome.eml").read
+      }
+    end
+
+    assert_response :unauthorized
+  end
+
+  test "raising when the configured Mailgun API key is nil" do
+    switch_key_to nil do
+      assert_raises ArgumentError do
+        travel_to "2018-10-09 15:15:00 EDT"
+        post rails_mailgun_inbound_emails_url, params: {
+          timestamp: 1539112500,
+          token: "7VwW7k6Ak7zcTwoSoNm7aTtbk1g67MKAnsYLfUB7PdszbgR5Xi",
+          signature: "ef24c5225322217bb065b80bb54eb4f9206d764e3e16abab07f0a64d1cf477cc",
+          "body-mime" => file_fixture("../files/welcome.eml").read
+        }
+      end
+    end
+  end
+
+  test "raising when the configured Mailgun API key is blank" do
+    switch_key_to "" do
+      assert_raises ArgumentError do
+        travel_to "2018-10-09 15:15:00 EDT"
+        post rails_mailgun_inbound_emails_url, params: {
+          timestamp: 1539112500,
+          token: "7VwW7k6Ak7zcTwoSoNm7aTtbk1g67MKAnsYLfUB7PdszbgR5Xi",
+          signature: "ef24c5225322217bb065b80bb54eb4f9206d764e3e16abab07f0a64d1cf477cc",
+          "body-mime" => file_fixture("../files/welcome.eml").read
+        }
+      end
+    end
+  end
+
+  private
+    def switch_key_to(new_key)
+      previous_key, ENV["MAILGUN_INGRESS_API_KEY"] = ENV["MAILGUN_INGRESS_API_KEY"], new_key
+      yield
+    ensure
+      ENV["MAILGUN_INGRESS_API_KEY"] = previous_key
+    end
+end
diff --git a/test/controllers/ingresses/mandrill/inbound_emails_controller_test.rb b/test/controllers/ingresses/mandrill/inbound_emails_controller_test.rb
new file mode 100644
index 0000000000..c8a8e731d6
--- /dev/null
+++ b/test/controllers/ingresses/mandrill/inbound_emails_controller_test.rb
@@ -0,0 +1,58 @@
+require "test_helper"
+
+ENV["MANDRILL_INGRESS_API_KEY"] = "1l9Qf7lutEf7h73VXfBwhw"
+
+class ActionMailbox::Ingresses::Mandrill::InboundEmailsControllerTest < ActionDispatch::IntegrationTest
+  setup do
+    ActionMailbox.ingress = :mandrill
+    @events = JSON.generate([{ event: "inbound", msg: { raw_msg: file_fixture("../files/welcome.eml").read } }])
+  end
+
+  test "receiving an inbound email from Mandrill" do
+    assert_difference -> { ActionMailbox::InboundEmail.count }, +1 do
+      post rails_mandrill_inbound_emails_url,
+        headers: { "X-Mandrill-Signature" => "gldscd2tAb/G+DmpiLcwukkLrC4=" }, params: { mandrill_events: @events }
+    end
+
+    assert_response :ok
+
+    inbound_email = ActionMailbox::InboundEmail.last
+    assert_equal file_fixture("../files/welcome.eml").read, inbound_email.raw_email.download
+    assert_equal "0CB459E0-0336-41DA-BC88-E6E28C697DDB@37signals.com", inbound_email.message_id
+  end
+
+  test "rejecting a forged inbound email from Mandrill" do
+    assert_no_difference -> { ActionMailbox::InboundEmail.count } do
+      post rails_mandrill_inbound_emails_url,
+        headers: { "X-Mandrill-Signature" => "forged" }, params: { mandrill_events: @events }
+    end
+
+    assert_response :unauthorized
+  end
+
+  test "raising when Mandrill API key is nil" do
+    switch_key_to nil do
+      assert_raises ArgumentError do
+        post rails_mandrill_inbound_emails_url,
+          headers: { "X-Mandrill-Signature" => "gldscd2tAb/G+DmpiLcwukkLrC4=" }, params: { mandrill_events: @events }
+      end
+    end
+  end
+
+  test "raising when Mandrill API key is blank" do
+    switch_key_to "" do
+      assert_raises ArgumentError do
+        post rails_mandrill_inbound_emails_url,
+          headers: { "X-Mandrill-Signature" => "gldscd2tAb/G+DmpiLcwukkLrC4=" }, params: { mandrill_events: @events }
+      end
+    end
+  end
+
+  private
+    def switch_key_to(new_key)
+      previous_key, ENV["MANDRILL_INGRESS_API_KEY"] = ENV["MANDRILL_INGRESS_API_KEY"], new_key
+      yield
+    ensure
+      ENV["MANDRILL_INGRESS_API_KEY"] = previous_key
+    end
+end
diff --git a/test/controllers/ingresses/postfix/inbound_emails_controller_test.rb b/test/controllers/ingresses/postfix/inbound_emails_controller_test.rb
new file mode 100644
index 0000000000..5e0777aa30
--- /dev/null
+++ b/test/controllers/ingresses/postfix/inbound_emails_controller_test.rb
@@ -0,0 +1,54 @@
+require "test_helper"
+
+class ActionMailbox::Ingresses::Postfix::InboundEmailsControllerTest < ActionDispatch::IntegrationTest
+  setup { ActionMailbox.ingress = :postfix }
+
+  test "receiving an inbound email from Postfix" do
+    assert_difference -> { ActionMailbox::InboundEmail.count }, +1 do
+      post rails_postfix_inbound_emails_url, headers: { "Authorization" => credentials, "Content-Type" => "message/rfc822" },
+        params: file_fixture("../files/welcome.eml").read
+    end
+
+    assert_response :no_content
+
+    inbound_email = ActionMailbox::InboundEmail.last
+    assert_equal file_fixture("../files/welcome.eml").read, inbound_email.raw_email.download
+    assert_equal "0CB459E0-0336-41DA-BC88-E6E28C697DDB@37signals.com", inbound_email.message_id
+  end
+
+  test "rejecting an unauthorized inbound email from Postfix" do
+    assert_no_difference -> { ActionMailbox::InboundEmail.count } do
+      post rails_postfix_inbound_emails_url, headers: { "Content-Type" => "message/rfc822" },
+        params: file_fixture("../files/welcome.eml").read
+    end
+
+    assert_response :unauthorized
+  end
+
+  test "rejecting an inbound email of an unsupported media type from Postfix" do
+    assert_no_difference -> { ActionMailbox::InboundEmail.count } do
+      post rails_postfix_inbound_emails_url, headers: { "Authorization" => credentials, "Content-Type" => "text/plain" },
+        params: file_fixture("../files/welcome.eml").read
+    end
+
+    assert_response :unsupported_media_type
+  end
+
+  test "raising when the configured password is nil" do
+    switch_password_to nil do
+      assert_raises ArgumentError do
+        post rails_postfix_inbound_emails_url, headers: { "Authorization" => credentials, "Content-Type" => "message/rfc822" },
+          params: file_fixture("../files/welcome.eml").read
+      end
+    end
+  end
+
+  test "raising when the configured password is blank" do
+    switch_password_to "" do
+      assert_raises ArgumentError do
+        post rails_postfix_inbound_emails_url, headers: { "Authorization" => credentials, "Content-Type" => "message/rfc822" },
+          params: file_fixture("../files/welcome.eml").read
+      end
+    end
+  end
+end
diff --git a/test/controllers/ingresses/sendgrid/inbound_emails_controller_test.rb b/test/controllers/ingresses/sendgrid/inbound_emails_controller_test.rb
new file mode 100644
index 0000000000..0c7d0d6846
--- /dev/null
+++ b/test/controllers/ingresses/sendgrid/inbound_emails_controller_test.rb
@@ -0,0 +1,44 @@
+require "test_helper"
+
+class ActionMailbox::Ingresses::Sendgrid::InboundEmailsControllerTest < ActionDispatch::IntegrationTest
+  setup { ActionMailbox.ingress = :sendgrid }
+
+  test "receiving an inbound email from Sendgrid" do
+    assert_difference -> { ActionMailbox::InboundEmail.count }, +1 do
+      post rails_sendgrid_inbound_emails_url,
+        headers: { authorization: credentials }, params: { email: file_fixture("../files/welcome.eml").read }
+    end
+
+    assert_response :no_content
+
+    inbound_email = ActionMailbox::InboundEmail.last
+    assert_equal file_fixture("../files/welcome.eml").read, inbound_email.raw_email.download
+    assert_equal "0CB459E0-0336-41DA-BC88-E6E28C697DDB@37signals.com", inbound_email.message_id
+  end
+
+  test "rejecting an unauthorized inbound email from Sendgrid" do
+    assert_no_difference -> { ActionMailbox::InboundEmail.count } do
+      post rails_sendgrid_inbound_emails_url, params: { email: file_fixture("../files/welcome.eml").read }
+    end
+
+    assert_response :unauthorized
+  end
+
+  test "raising when the configured password is nil" do
+    switch_password_to nil do
+      assert_raises ArgumentError do
+        post rails_sendgrid_inbound_emails_url,
+          headers: { authorization: credentials }, params: { email: file_fixture("../files/welcome.eml").read }
+      end
+    end
+  end
+
+  test "raising when the configured password is blank" do
+    switch_password_to "" do
+      assert_raises ArgumentError do
+        post rails_sendgrid_inbound_emails_url,
+          headers: { authorization: credentials }, params: { email: file_fixture("../files/welcome.eml").read }
+      end
+    end
+  end
+end
diff --git a/test/fixtures/files/text.txt b/test/fixtures/files/text.txt
deleted file mode 100644
index 84c3f1cf21..0000000000
--- a/test/fixtures/files/text.txt
+++ /dev/null
@@ -1 +0,0 @@
-This Is Not An Email!
diff --git a/test/test_helper.rb b/test/test_helper.rb
index 264f9e8482..b4459f3feb 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -1,5 +1,5 @@
-# Configure Rails Environment
 ENV["RAILS_ENV"] = "test"
+ENV["RAILS_INBOUND_EMAIL_PASSWORD"] = "tbsy84uSV1Kt3ZJZELY2TmShPRs91E3yL4tzf96297vBCkDWgL"
 
 require_relative "../test/dummy/config/environment"
 ActiveRecord::Migrator.migrations_paths = [File.expand_path("../test/dummy/db/migrate", __dir__)]
@@ -7,14 +7,11 @@ require "rails/test_help"
 
 require "byebug"
 
-# Filter out Minitest backtrace while allowing backtrace from other libraries
-# to be shown.
 Minitest.backtrace_filter = Minitest::BacktraceFilter.new
 
 require "rails/test_unit/reporter"
 Rails::TestUnitReporter.executable = 'bin/test'
 
-# Load fixtures from the engine
 if ActiveSupport::TestCase.respond_to?(:fixture_path=)
   ActiveSupport::TestCase.fixture_path = File.expand_path("fixtures", __dir__)
   ActionDispatch::IntegrationTest.fixture_path = ActiveSupport::TestCase.fixture_path
@@ -28,6 +25,20 @@ class ActiveSupport::TestCase
   include ActionMailbox::TestHelper, ActiveJob::TestHelper
 end
 
+class ActionDispatch::IntegrationTest
+  private
+    def credentials
+      ActionController::HttpAuthentication::Basic.encode_credentials "actionmailbox", ENV["RAILS_INBOUND_EMAIL_PASSWORD"]
+    end
+
+    def switch_password_to(new_password)
+      previous_password, ENV["RAILS_INBOUND_EMAIL_PASSWORD"] = ENV["RAILS_INBOUND_EMAIL_PASSWORD"], new_password
+      yield
+    ensure
+      ENV["RAILS_INBOUND_EMAIL_PASSWORD"] = previous_password
+    end
+end
+
 if ARGV.include?("-v")
   ActiveRecord::Base.logger = Logger.new(STDOUT)
   ActiveJob::Base.logger    = Logger.new(STDOUT)
diff --git a/test/unit/controller_test.rb b/test/unit/controller_test.rb
deleted file mode 100644
index 508e561244..0000000000
--- a/test/unit/controller_test.rb
+++ /dev/null
@@ -1,27 +0,0 @@
-require_relative '../test_helper'
-
-class ActionMailbox::InboundEmailsControllerTest < ActionDispatch::IntegrationTest
-  test "receiving a valid RFC 822 message" do
-    assert_difference -> { ActionMailbox::InboundEmail.count }, +1 do
-      post_inbound_email "welcome.eml"
-    end
-
-    assert_response :created
-
-    inbound_email = ActionMailbox::InboundEmail.last
-    assert_equal file_fixture('../files/welcome.eml').read, inbound_email.raw_email.download
-  end
-
-  test "rejecting a message of an unsupported type" do
-    assert_no_difference -> { ActionMailbox::InboundEmail.count } do
-      post rails_inbound_emails_url, params: { message: fixture_file_upload('files/text.txt', 'text/plain') }
-    end
-
-    assert_response :unsupported_media_type
-  end
-
-  private
-    def post_inbound_email(fixture_name)
-      post rails_inbound_emails_url, params: { message: fixture_file_upload("files/#{fixture_name}", 'message/rfc822') }
-    end
-end
diff --git a/test/unit/inbound_email/message_id_test.rb b/test/unit/inbound_email/message_id_test.rb
index aa9ce90b4c..c744a5bf99 100644
--- a/test/unit/inbound_email/message_id_test.rb
+++ b/test/unit/inbound_email/message_id_test.rb
@@ -7,9 +7,7 @@ class ActionMailbox::InboundEmail::MessageIdTest < ActiveSupport::TestCase
   end
 
   test "message id is generated if its missing" do
-    source_without_message_id = "Date: Fri, 28 Sep 2018 11:08:55 -0700\r\nTo: a@example.com\r\nMime-Version: 1.0\r\nContent-Type: text/plain\r\nContent-Transfer-Encoding: 7bit\r\n\r\nHello!"
-    inbound_email = create_inbound_email Tempfile.new.tap { |raw_email| raw_email.write source_without_message_id }
-
+    inbound_email = create_inbound_email "Date: Fri, 28 Sep 2018 11:08:55 -0700\r\nTo: a@example.com\r\nMime-Version: 1.0\r\nContent-Type: text/plain\r\nContent-Transfer-Encoding: 7bit\r\n\r\nHello!"
     assert_not_nil inbound_email.message_id
   end
 end