From 04cfc8ed9d89c041aa457c3f5f2227ff32787671 Mon Sep 17 00:00:00 2001
From: Pierre de La Morinerie <kemenaran@gmail.com>
Date: Tue, 18 Jan 2022 16:56:30 +0100
Subject: [PATCH 1/3] config: use alternate delivery methods to configure
 ActionMailer

Previously `SENDINBLUE_BALANCING` was used only when
`SENDINBLUE_ENABLED` was *disabled* (otherwise only SendInBlue was ever
used).

This commit:

- Ensure that `SENDINBLUE_BALANCING` is used only when SendInBlue is
  *enabled* (which is more intuitive).
- Make it easier to add other delivery methods.
---
 app/lib/mailtrap/smtp.rb                      |  4 +++
 app/lib/sendinblue/smtp.rb                    |  4 +++
 config/environments/production.rb             | 33 +++++++++++--------
 .../dynamic_smtp_settings_interceptor.rb      | 26 ++++++++-------
 4 files changed, 43 insertions(+), 24 deletions(-)
 create mode 100644 app/lib/mailtrap/smtp.rb
 create mode 100644 app/lib/sendinblue/smtp.rb

diff --git a/app/lib/mailtrap/smtp.rb b/app/lib/mailtrap/smtp.rb
new file mode 100644
index 000000000..93b57f412
--- /dev/null
+++ b/app/lib/mailtrap/smtp.rb
@@ -0,0 +1,4 @@
+module Mailtrap
+  class Smtp < ::Mail::SMTP
+  end
+end
diff --git a/app/lib/sendinblue/smtp.rb b/app/lib/sendinblue/smtp.rb
new file mode 100644
index 000000000..cccb4872e
--- /dev/null
+++ b/app/lib/sendinblue/smtp.rb
@@ -0,0 +1,4 @@
+module Sendinblue
+  class Smtp < ::Mail::SMTP
+  end
+end
diff --git a/config/environments/production.rb b/config/environments/production.rb
index cdfeaed9b..b006c9ec3 100644
--- a/config/environments/production.rb
+++ b/config/environments/production.rb
@@ -1,4 +1,6 @@
 require "active_support/core_ext/integer/time"
+require Rails.root.join("app/lib/mailtrap/smtp")
+require Rails.root.join("app/lib/sendinblue/smtp")
 
 Rails.application.configure do
   # Settings specified here will take precedence over those in config/application.rb.
@@ -76,8 +78,8 @@ Rails.application.configure do
   # config.action_mailer.raise_delivery_errors = false
 
   if ENV['MAILTRAP_ENABLED'] == 'enabled'
-    config.action_mailer.delivery_method = :smtp
-    config.action_mailer.smtp_settings = {
+    ActionMailer::Base.add_delivery_method :mailtrap, Mailtrap::Smtp
+    config.action_mailer.mailtrap_settings = {
       user_name: Rails.application.secrets.mailtrap[:username],
       password: Rails.application.secrets.mailtrap[:password],
       address: 'smtp.mailtrap.io',
@@ -85,17 +87,22 @@ Rails.application.configure do
       port: '2525',
       authentication: :cram_md5
     }
-  elsif ENV['SENDINBLUE_ENABLED'] == 'enabled'
-    config.action_mailer.delivery_method = :smtp
-    config.action_mailer.smtp_settings = {
-      user_name: Rails.application.secrets.sendinblue[:username],
-      password: Rails.application.secrets.sendinblue[:smtp_key],
-      address: 'smtp-relay.sendinblue.com',
-      domain: 'smtp-relay.sendinblue.com',
-      port: '587',
-      authentication: :cram_md5
-    }
-  else
+    config.action_mailer.delivery_method = :mailtrap
+  elsif
+    if ENV['SENDINBLUE_ENABLED'] == 'enabled'
+      ActionMailer::Base.add_delivery_method :sendinblue, Sendinblue::Smtp
+      config.action_mailer.sendinblue_settings = {
+        user_name: Rails.application.secrets.sendinblue[:username],
+        password: Rails.application.secrets.sendinblue[:smtp_key],
+        address: 'smtp-relay.sendinblue.com',
+        domain: 'smtp-relay.sendinblue.com',
+        port: '587',
+        authentication: :cram_md5
+      }
+    end
+
+    # Default delivery method
+    # (Actual delivery method will be selected at runtime by DynamicSmtpSettingsInterceptor)
     config.action_mailer.delivery_method = :mailjet
   end
 
diff --git a/lib/action_mailer/dynamic_smtp_settings_interceptor.rb b/lib/action_mailer/dynamic_smtp_settings_interceptor.rb
index 91d6398f8..2aeef5c5e 100644
--- a/lib/action_mailer/dynamic_smtp_settings_interceptor.rb
+++ b/lib/action_mailer/dynamic_smtp_settings_interceptor.rb
@@ -6,17 +6,21 @@
 
 class DynamicSmtpSettingsInterceptor
   def self.delivering_email(message)
-    if ENV['SENDINBLUE_BALANCING'] == 'enabled'
-      if rand(0..99) < ENV['SENDINBLUE_BALANCING_VALUE'].to_i
-        message.delivery_method.settings = {
-          user_name: ENV['SENDINBLUE_USER_NAME'],
-          password: ENV['SENDINBLUE_SMTP_KEY'],
-          address: 'smtp-relay.sendinblue.com',
-          domain: 'smtp-relay.sendinblue.com',
-          port: '587',
-          authentication: :cram_md5
-        }
-      end
+    if balance_to_sendinblue?
+      ApplicationMailer.wrap_delivery_behavior(message, :sendinblue)
+    end
+    # fallback to the default delivery method
+  end
+
+  private
+
+  def self.balance_to_sendinblue?
+    if ENV.fetch('SENDINBLUE_ENABLED') != 'enabled'
+      false
+    elsif ENV.fetch('SENDINBLUE_BALANCING') == 'enabled'
+      rand(0..99) < ENV.fetch('SENDINBLUE_BALANCING_VALUE').to_i
+    else
+      true
     end
   end
 end

From 27b42fe8ae9424f1b1f567737960b284698efaf0 Mon Sep 17 00:00:00 2001
From: Pierre de La Morinerie <kemenaran@gmail.com>
Date: Wed, 19 Jan 2022 20:30:01 +0000
Subject: [PATCH 2/3] config: better document `SENDINBLUE_BALANCING`

---
 config/env.example | 27 ++++++++++++++++-----------
 1 file changed, 16 insertions(+), 11 deletions(-)

diff --git a/config/env.example b/config/env.example
index d2f9de017..8f3e332f8 100644
--- a/config/env.example
+++ b/config/env.example
@@ -70,28 +70,33 @@ MATOMO_ENABLED="disabled"
 MATOMO_ID=""
 MATOMO_HOST="matomo.organisme.fr"
 
-# SMTP Provider: Send In Blue
+# Default SMTP Provider: Mailjet
+MAILJET_API_KEY=""
+MAILJET_SECRET_KEY=""
+
+# Alternate SMTP Provider: SendInBlue
 SENDINBLUE_ENABLED="disabled"
-SENDINBLUE_BALANCING=""
-SENDINBLUE_BALANCING_VALUE=""
 SENDINBLUE_CLIENT_KEY=""
 SENDINBLUE_SMTP_KEY=""
 SENDINBLUE_USER_NAME=""
 # SENDINBLUE_LOGIN_URL="https://app.sendinblue.com/account/saml/login/truc"
 
-# SMTP Provider: Mailjet
-MAILJET_API_KEY=""
-MAILJET_SECRET_KEY=""
+# Ratio of emails sent using SendInBlue
+# When enabled, N % of emails will be sent using SendInBlue
+# (and the others using the default SMTP provider)
+SENDINBLUE_BALANCING="disabled"
+SENDINBLUE_BALANCING_VALUE="50"
+
+# Alternate SMTP Provider: Mailtrap (mail catcher for staging environments)
+# When enabled, all emails will be sent using this provided
+MAILTRAP_ENABLED="disabled"
+MAILTRAP_USERNAME=""
+MAILTRAP_PASSWORD=""
 
 # External service: live chat for admins (specific to démarches-simplifiées.fr)
 CRISP_ENABLED="disabled"
 CRISP_CLIENT_KEY=""
 
-# External service: mail catcher for staging environments (specific to démarches-simplifiées.fr)
-MAILTRAP_ENABLED="disabled"
-MAILTRAP_USERNAME=""
-MAILTRAP_PASSWORD=""
-
 # API Entreprise credentials
 # https://api.gouv.fr/api/api-entreprise.html
 API_ENTREPRISE_KEY=""

From 847abca1226466aabc31eaf795ed92a61d3c7fae Mon Sep 17 00:00:00 2001
From: Pierre de La Morinerie <kemenaran@gmail.com>
Date: Wed, 26 Jan 2022 10:33:12 +0000
Subject: [PATCH 3/3] config: simplify mailer configuration again

Move everything to initializers, and replace the email settings
interceptor by a BalancerDeliveryMethod.

It has the advantage that it can be configured entirely from the
`config/environment.rb` file, without an extra file to look at.
---
 app/lib/balancer_delivery_method.rb           | 38 ++++++++
 app/lib/mailtrap/smtp.rb                      |  4 -
 app/lib/sendinblue/smtp.rb                    |  4 -
 config/environments/development.rb            | 39 ++------
 config/environments/production.rb             | 38 +++-----
 .../dynamic_smtp_settings_interceptor.rb      | 14 ---
 config/initializers/helo.rb                   | 17 ++++
 config/initializers/mailtrap.rb               | 17 ++++
 config/initializers/sendinblue.rb             | 24 ++++-
 .../dynamic_smtp_settings_interceptor.rb      | 26 ------
 spec/lib/balancer_delivery_method_spec.rb     | 93 +++++++++++++++++++
 11 files changed, 205 insertions(+), 109 deletions(-)
 create mode 100644 app/lib/balancer_delivery_method.rb
 delete mode 100644 app/lib/mailtrap/smtp.rb
 delete mode 100644 app/lib/sendinblue/smtp.rb
 delete mode 100644 config/initializers/dynamic_smtp_settings_interceptor.rb
 create mode 100644 config/initializers/helo.rb
 create mode 100644 config/initializers/mailtrap.rb
 delete mode 100644 lib/action_mailer/dynamic_smtp_settings_interceptor.rb
 create mode 100644 spec/lib/balancer_delivery_method_spec.rb

diff --git a/app/lib/balancer_delivery_method.rb b/app/lib/balancer_delivery_method.rb
new file mode 100644
index 000000000..85f468ae3
--- /dev/null
+++ b/app/lib/balancer_delivery_method.rb
@@ -0,0 +1,38 @@
+# A Mail delivery method that randomly balances the actual delivery between different
+# methods.
+#
+# Usage:
+#
+# ```ruby
+#   ActionMailer::Base.add_delivery_method :balancer, BalancerDeliveryMethod
+#   config.action_mailer.balancer_settings = {
+#     smtp: 25,
+#     sendmail: 75
+#   }
+#   config.action_mailer.delivery_method = :balancer
+# ```
+#
+# Be sure to restart your server when you modify this file.
+class BalancerDeliveryMethod
+  # Allows configuring the random number generator used for selecting a delivery method,
+  # mostly for testing purposes.
+  mattr_accessor :random, default: Random.new
+
+  def initialize(settings)
+    @delivery_methods = settings
+  end
+
+  def deliver!(mail)
+    balanced_delivery_method = delivery_method(mail)
+    ApplicationMailer.wrap_delivery_behavior(mail, balanced_delivery_method)
+    mail.deliver
+  end
+
+  private
+
+  def delivery_method(mail)
+    @delivery_methods
+      .flat_map { |delivery_method, weight| [delivery_method] * weight }
+      .sample(random: self.class.random)
+  end
+end
diff --git a/app/lib/mailtrap/smtp.rb b/app/lib/mailtrap/smtp.rb
deleted file mode 100644
index 93b57f412..000000000
--- a/app/lib/mailtrap/smtp.rb
+++ /dev/null
@@ -1,4 +0,0 @@
-module Mailtrap
-  class Smtp < ::Mail::SMTP
-  end
-end
diff --git a/app/lib/sendinblue/smtp.rb b/app/lib/sendinblue/smtp.rb
deleted file mode 100644
index cccb4872e..000000000
--- a/app/lib/sendinblue/smtp.rb
+++ /dev/null
@@ -1,4 +0,0 @@
-module Sendinblue
-  class Smtp < ::Mail::SMTP
-  end
-end
diff --git a/config/environments/development.rb b/config/environments/development.rb
index 2b0f92cbc..e0be95cc4 100644
--- a/config/environments/development.rb
+++ b/config/environments/development.rb
@@ -75,40 +75,13 @@ Rails.application.configure do
   config.assets.raise_runtime_errors = true
 
   # Action Mailer settings
+  config.action_mailer.delivery_method = :letter_opener
 
-  if ENV['SENDINBLUE_ENABLED'] == 'enabled'
-    config.action_mailer.delivery_method = :smtp
-    config.action_mailer.smtp_settings = {
-      user_name: Rails.application.secrets.sendinblue[:username],
-      password: Rails.application.secrets.sendinblue[:smtp_key],
-      address: 'smtp-relay.sendinblue.com',
-      domain: 'smtp-relay.sendinblue.com',
-      port: '587',
-      authentication: :cram_md5
-    }
-  else
-    # https://usehelo.com
-    if ENV['HELO_ENABLED'] == 'enabled'
-      config.action_mailer.delivery_method = :smtp
-      config.action_mailer.smtp_settings = {
-        user_name: 'demarches-simplifiees',
-        password: '',
-        address: '127.0.0.1',
-        domain: '127.0.0.1',
-        port: ENV.fetch('HELO_PORT', '2525'),
-        authentication: :plain
-      }
-    else
-      config.action_mailer.delivery_method = :letter_opener_web
-    end
-
-    config.action_mailer.default_url_options = {
-      host: 'localhost',
-      port: 3000
-    }
-
-    config.action_mailer.asset_host = "http://" + ENV['APP_HOST']
-  end
+  config.action_mailer.default_url_options = {
+    host: 'localhost',
+    port: 3000
+  }
+  config.action_mailer.asset_host = "http://" + ENV['APP_HOST']
 
   Rails.application.routes.default_url_options = {
     host: 'localhost',
diff --git a/config/environments/production.rb b/config/environments/production.rb
index b006c9ec3..912426727 100644
--- a/config/environments/production.rb
+++ b/config/environments/production.rb
@@ -1,6 +1,5 @@
 require "active_support/core_ext/integer/time"
-require Rails.root.join("app/lib/mailtrap/smtp")
-require Rails.root.join("app/lib/sendinblue/smtp")
+require Rails.root.join("app/lib/balancer_delivery_method")
 
 Rails.application.configure do
   # Settings specified here will take precedence over those in config/application.rb.
@@ -78,31 +77,20 @@ Rails.application.configure do
   # config.action_mailer.raise_delivery_errors = false
 
   if ENV['MAILTRAP_ENABLED'] == 'enabled'
-    ActionMailer::Base.add_delivery_method :mailtrap, Mailtrap::Smtp
-    config.action_mailer.mailtrap_settings = {
-      user_name: Rails.application.secrets.mailtrap[:username],
-      password: Rails.application.secrets.mailtrap[:password],
-      address: 'smtp.mailtrap.io',
-      domain: 'smtp.mailtrap.io',
-      port: '2525',
-      authentication: :cram_md5
-    }
     config.action_mailer.delivery_method = :mailtrap
-  elsif
-    if ENV['SENDINBLUE_ENABLED'] == 'enabled'
-      ActionMailer::Base.add_delivery_method :sendinblue, Sendinblue::Smtp
-      config.action_mailer.sendinblue_settings = {
-        user_name: Rails.application.secrets.sendinblue[:username],
-        password: Rails.application.secrets.sendinblue[:smtp_key],
-        address: 'smtp-relay.sendinblue.com',
-        domain: 'smtp-relay.sendinblue.com',
-        port: '587',
-        authentication: :cram_md5
-      }
-    end
 
-    # Default delivery method
-    # (Actual delivery method will be selected at runtime by DynamicSmtpSettingsInterceptor)
+  elsif ENV['SENDINBLUE_ENABLED'] == 'enabled' && ENV['SENDINBLUE_BALANCING'] == 'enabled'
+    ActionMailer::Base.add_delivery_method :balancer, BalancerDeliveryMethod
+    config.action_mailer.balancer_settings = {
+      sendinblue: ENV.fetch('SENDINBLUE_BALANCING_VALUE').to_i,
+      mailjet: 100 - ENV.fetch('SENDINBLUE_BALANCING_VALUE').to_i
+    }
+    config.action_mailer.delivery_method = :balancer
+
+  elsif ENV['SENDINBLUE_ENABLED'] == 'enabled'
+    config.action_mailer.delivery_method = :sendinblue
+
+  else
     config.action_mailer.delivery_method = :mailjet
   end
 
diff --git a/config/initializers/dynamic_smtp_settings_interceptor.rb b/config/initializers/dynamic_smtp_settings_interceptor.rb
deleted file mode 100644
index b91ee7e19..000000000
--- a/config/initializers/dynamic_smtp_settings_interceptor.rb
+++ /dev/null
@@ -1,14 +0,0 @@
-# We want to register an interceptor, but we can't make the action idempotent
-# (because there's no way to peek at the currently registered interceptors).
-#
-# To make zeitwerk happy, instead signal that we don't want the
-# DynamicSmtpSettingsInterceptor constant to be auto-loaded, by:
-# - adding it to a non-autoloaded-path (/lib),
-# - requiring it explicitely.
-#
-# See https://guides.rubyonrails.org/autoloading_and_reloading_constants.html#autoloading-when-the-application-boots
-require 'action_mailer/dynamic_smtp_settings_interceptor'
-
-ActiveSupport.on_load(:action_mailer) do
-  ActionMailer::Base.register_interceptor DynamicSmtpSettingsInterceptor
-end
diff --git a/config/initializers/helo.rb b/config/initializers/helo.rb
new file mode 100644
index 000000000..ed119ba81
--- /dev/null
+++ b/config/initializers/helo.rb
@@ -0,0 +1,17 @@
+if ENV['HELO_ENABLED'] == 'enabled'
+  ActiveSupport.on_load(:action_mailer) do
+    module Helo
+      class SMTP < ::Mail::SMTP; end
+    end
+
+    ActionMailer::Base.add_delivery_method :helo, Helo::SMTP
+    ActionMailer::Base.helo_settings = {
+      user_name: 'demarches-simplifiees',
+      password: '',
+      address: '127.0.0.1',
+      domain: '127.0.0.1',
+      port: ENV.fetch('HELO_PORT', '2525'),
+      authentication: :plain
+    }
+  end
+end
diff --git a/config/initializers/mailtrap.rb b/config/initializers/mailtrap.rb
new file mode 100644
index 000000000..6d1faa04b
--- /dev/null
+++ b/config/initializers/mailtrap.rb
@@ -0,0 +1,17 @@
+if ENV.fetch('MAILTRAP_ENABLED') == 'enabled'
+  ActiveSupport.on_load(:action_mailer) do
+    module Mailtrap
+      class SMTP < ::Mail::SMTP; end
+    end
+
+    ActionMailer::Base.add_delivery_method :mailtrap, Mailtrap::SMTP
+    ActionMailer::Base.mailtrap_settings = {
+      user_name: Rails.application.secrets.mailtrap[:username],
+      password: Rails.application.secrets.mailtrap[:password],
+      address: 'smtp.mailtrap.io',
+      domain: 'smtp.mailtrap.io',
+      port: '2525',
+      authentication: :cram_md5
+    }
+  end
+end
diff --git a/config/initializers/sendinblue.rb b/config/initializers/sendinblue.rb
index d523b2373..206b3905d 100644
--- a/config/initializers/sendinblue.rb
+++ b/config/initializers/sendinblue.rb
@@ -1,5 +1,23 @@
-require 'sib-api-v3-sdk'
+if ENV.fetch('SENDINBLUE_ENABLED') == 'enabled'
+  require 'sib-api-v3-sdk'
 
-SibApiV3Sdk.configure do |config|
-  config.api_key['api-key'] = ENV.fetch('SENDINBLUE_API_V3_KEY', '')
+  ActiveSupport.on_load(:action_mailer) do
+    module Sendinblue
+      class SMTP < ::Mail::SMTP; end
+    end
+
+    ActionMailer::Base.add_delivery_method :sendinblue, Sendinblue::SMTP
+    ActionMailer::Base.sendinblue_settings = {
+      user_name: Rails.application.secrets.sendinblue[:username],
+      password: Rails.application.secrets.sendinblue[:smtp_key],
+      address: 'smtp-relay.sendinblue.com',
+      domain: 'smtp-relay.sendinblue.com',
+      port: '587',
+      authentication: :cram_md5
+    }
+  end
+
+  SibApiV3Sdk.configure do |config|
+    config.api_key['api-key'] = Rails.application.secrets.sendinblue[:api_v3_key]
+  end
 end
diff --git a/lib/action_mailer/dynamic_smtp_settings_interceptor.rb b/lib/action_mailer/dynamic_smtp_settings_interceptor.rb
deleted file mode 100644
index 2aeef5c5e..000000000
--- a/lib/action_mailer/dynamic_smtp_settings_interceptor.rb
+++ /dev/null
@@ -1,26 +0,0 @@
-# Note: this class is instanciated when being added as an interceptor
-# during the app initialization.
-#
-# If you edit this file in development env, you will need to restart
-# the app to see the changes.
-
-class DynamicSmtpSettingsInterceptor
-  def self.delivering_email(message)
-    if balance_to_sendinblue?
-      ApplicationMailer.wrap_delivery_behavior(message, :sendinblue)
-    end
-    # fallback to the default delivery method
-  end
-
-  private
-
-  def self.balance_to_sendinblue?
-    if ENV.fetch('SENDINBLUE_ENABLED') != 'enabled'
-      false
-    elsif ENV.fetch('SENDINBLUE_BALANCING') == 'enabled'
-      rand(0..99) < ENV.fetch('SENDINBLUE_BALANCING_VALUE').to_i
-    else
-      true
-    end
-  end
-end
diff --git a/spec/lib/balancer_delivery_method_spec.rb b/spec/lib/balancer_delivery_method_spec.rb
new file mode 100644
index 000000000..b44fbdad8
--- /dev/null
+++ b/spec/lib/balancer_delivery_method_spec.rb
@@ -0,0 +1,93 @@
+RSpec.describe BalancerDeliveryMethod do
+  class ExampleMailer < ApplicationMailer
+    def greet(name)
+      mail(to: "smtp_to", from: "smtp_from", body: "Hello #{name}")
+    end
+  end
+
+  class TestMail
+    def self.deliveries
+      @deliveries ||= []
+    end
+
+    def self.deliveries=(val)
+      @deliveries = val
+    end
+
+    attr_accessor :settings
+
+    def initialize(values)
+      @settings = values.dup
+    end
+
+    def deliver!(mail)
+      Mail::CheckDeliveryParams.check(mail)
+      self.class.deliveries << mail
+    end
+  end
+
+  class MockSmtp < TestMail; end
+
+  class MockSendmail < TestMail; end
+
+  class FixedSequence
+    def initialize(sequence)
+      @enumerator = sequence.each
+    end
+
+    def rand(_)
+      @enumerator.next
+    end
+  end
+
+  before do
+    ActionMailer::Base.add_delivery_method :mock_smtp, MockSmtp
+    ActionMailer::Base.add_delivery_method :mock_sendmail, MockSendmail
+    ActionMailer::Base.add_delivery_method :balancer, BalancerDeliveryMethod
+
+    ExampleMailer.delivery_method = :balancer
+  end
+
+  context 'when a single delivery method is provided' do
+    before do
+      ActionMailer::Base.balancer_settings = { mock_smtp: 10 }
+    end
+
+    it 'sends emails to the selected delivery method' do
+      mail = ExampleMailer.greet('Joshua').deliver_now
+      expect(mail).to have_been_delivered_using(MockSmtp)
+    end
+  end
+
+  context 'when multiple delivery methods are provided' do
+    before do
+      ActionMailer::Base.balancer_settings = { mock_smtp: 10, mock_sendmail: 5 }
+
+      rng_sequence = [3, 14, 1]
+      BalancerDeliveryMethod.random = FixedSequence.new(rng_sequence)
+    end
+
+    after do
+      BalancerDeliveryMethod.random = Random.new
+    end
+
+    it 'sends emails randomly, given the provided weights' do
+      mail1 = ExampleMailer.greet('Lucia').deliver_now
+      expect(mail1).to have_been_delivered_using(MockSmtp)
+
+      mail2 = ExampleMailer.greet('Damian').deliver_now
+      expect(mail2).to have_been_delivered_using(MockSendmail)
+
+      mail3 = ExampleMailer.greet('Rahwa').deliver_now
+      expect(mail3).to have_been_delivered_using(MockSmtp)
+    end
+  end
+
+  # Helpers
+
+  def have_been_delivered_using(delivery_class)
+    satisfy("have been delivered using #{delivery_class}") do |mail|
+      delivery_class.deliveries.include?(mail)
+    end
+  end
+end