diff --git a/actionmailer/lib/action_mailer/test_case.rb b/actionmailer/lib/action_mailer/test_case.rb
index d4874c6dbf..b91eed592a 100644
--- a/actionmailer/lib/action_mailer/test_case.rb
+++ b/actionmailer/lib/action_mailer/test_case.rb
@@ -8,55 +8,69 @@ module ActionMailer
   end
 
   class TestCase < ActiveSupport::TestCase
-    include TestHelper
+    module Behavior
+      extend ActiveSupport::Concern
 
-    setup :initialize_test_deliveries
-    setup :set_expected_mail
+      include TestHelper
 
-    class << self
-      def tests(mailer)
-        write_inheritable_attribute(:mailer_class, mailer)
-      end
+      module ClassMethods
+        def tests(mailer)
+          write_inheritable_attribute(:mailer_class, mailer)
+        end
 
-      def mailer_class
-        if mailer = read_inheritable_attribute(:mailer_class)
-          mailer
-        else
-          tests determine_default_mailer(name)
+        def mailer_class
+          if mailer = read_inheritable_attribute(:mailer_class)
+            mailer
+          else
+            tests determine_default_mailer(name)
+          end
+        end
+
+        def determine_default_mailer(name)
+          name.sub(/Test$/, '').constantize
+        rescue NameError => e
+          raise NonInferrableMailerError.new(name)
         end
       end
 
-      def determine_default_mailer(name)
-        name.sub(/Test$/, '').constantize
-      rescue NameError => e
-        raise NonInferrableMailerError.new(name)
+      module InstanceMethods
+
+      protected
+
+        def initialize_test_deliveries
+          ActionMailer::Base.delivery_method = :test
+          ActionMailer::Base.perform_deliveries = true
+          ActionMailer::Base.deliveries.clear
+        end
+
+        def set_expected_mail
+          @expected = Mail.new
+          @expected.content_type ["text", "plain", { "charset" => charset }]
+          @expected.mime_version = '1.0'
+        end
+
+      private
+
+        def charset
+          "UTF-8"
+        end
+
+        def encode(subject)
+          Mail::Encodings.q_value_encode(subject, charset)
+        end
+
+        def read_fixture(action)
+          IO.readlines(File.join(Rails.root, 'test', 'fixtures', self.class.mailer_class.name.underscore, action))
+        end
+      end
+
+      included do
+        setup :initialize_test_deliveries
+        setup :set_expected_mail
       end
     end
 
-    protected
-      def initialize_test_deliveries
-        ActionMailer::Base.delivery_method = :test
-        ActionMailer::Base.perform_deliveries = true
-        ActionMailer::Base.deliveries.clear
-      end
+    include Behavior
 
-      def set_expected_mail
-        @expected = Mail.new
-        @expected.content_type ["text", "plain", { "charset" => charset }]
-        @expected.mime_version = '1.0'
-      end
-
-    private
-      def charset
-        "UTF-8"
-      end
-
-      def encode(subject)
-        Mail::Encodings.q_value_encode(subject, charset)
-      end
-
-      def read_fixture(action)
-        IO.readlines(File.join(Rails.root, 'test', 'fixtures', self.class.mailer_class.name.underscore, action))
-      end
   end
 end
diff --git a/actionmailer/lib/action_mailer/test_helper.rb b/actionmailer/lib/action_mailer/test_helper.rb
index 3a1612442f..5beab87ad2 100644
--- a/actionmailer/lib/action_mailer/test_helper.rb
+++ b/actionmailer/lib/action_mailer/test_helper.rb
@@ -1,5 +1,7 @@
 module ActionMailer
   module TestHelper
+    extend ActiveSupport::Concern
+
     # Asserts that the number of emails sent matches the given number.
     #
     #   def test_emails
@@ -57,11 +59,3 @@ module ActionMailer
     end
   end
 end
-
-module Test
-  module Unit
-    class TestCase
-      include ActionMailer::TestHelper
-    end
-  end
-end
diff --git a/actionmailer/test/old_base/mail_service_test.rb b/actionmailer/test/old_base/mail_service_test.rb
index e8e8fcedc9..527b37218a 100644
--- a/actionmailer/test/old_base/mail_service_test.rb
+++ b/actionmailer/test/old_base/mail_service_test.rb
@@ -1113,6 +1113,8 @@ class InheritableTemplateRootTest < ActiveSupport::TestCase
 end
 
 class MethodNamingTest < ActiveSupport::TestCase
+  include ActionMailer::TestHelper
+
   class TestMailer < ActionMailer::Base
     def send
       body       'foo'