8 files changed, 62 insertions, 14 deletions
diff --git a/actionmailer/CHANGELOG.md b/actionmailer/CHANGELOG.md
index 604e332dad..1fef9eae29 100644
--- a/actionmailer/CHANGELOG.md
+++ b/actionmailer/CHANGELOG.md
@@ -1,8 +1,30 @@
-## Rails 5.0.0.beta3 (February 24, 2016) ##
+*   Disallow calling `#deliver_later` after making local modifications to
+    the message which would be lost when the delivery job is enqueued.
+
+    Prevents a common, hard-to-find bug like
+
+        message = Notifier.welcome(user, foo)
+        message.message_id = my_generated_message_id
+        message.deliver_later
+
+    The message_id is silently lost! *Only the mailer arguments are passed
+    to the delivery job.*
+
+    This raises an exception now. Make modifications to the message within
+    the mailer method instead, or use a custom Active Job to manage delivery
+    instead of using #deliver_later.
 
-*   Add support to fragment cache in Action Mailer.
+    *Jeremy Daer*
+
+*   Removes `-t` from default Sendmail arguments to match the underlying
+    `Mail::Sendmail` setting.
+
+    *Clayton Liggitt*
+
+
+## Rails 5.0.0.beta3 (February 24, 2016) ##
 
-    Now you can use fragment caching in your mailers views.
+*   Add support for fragment caching in Action Mailer views.
 
     *Stan Lo*
 
diff --git a/actionmailer/lib/action_mailer/delivery_methods.rb b/actionmailer/lib/action_mailer/delivery_methods.rb
index 4758b55a2a..571c8e7d2a 100644
--- a/actionmailer/lib/action_mailer/delivery_methods.rb
+++ b/actionmailer/lib/action_mailer/delivery_methods.rb
@@ -36,7 +36,7 @@ module ActionMailer
 
       add_delivery_method :sendmail, Mail::Sendmail,
         location:  '/usr/sbin/sendmail',
-        arguments: '-i -t'
+        arguments: '-i'
 
       add_delivery_method :test, Mail::TestMailer
     end
diff --git a/actionmailer/lib/action_mailer/message_delivery.rb b/actionmailer/lib/action_mailer/message_delivery.rb
index 5fcb5a0c88..d638057d72 100644
--- a/actionmailer/lib/action_mailer/message_delivery.rb
+++ b/actionmailer/lib/action_mailer/message_delivery.rb
@@ -18,6 +18,7 @@ module ActionMailer
       @mailer = mailer
       @mail_method = mail_method
       @args = args
+      @obj = nil
     end
 
     def __getobj__ #:nodoc:
@@ -91,8 +92,19 @@ module ActionMailer
     private
 
       def enqueue_delivery(delivery_method, options={})
-        args = @mailer.name, @mail_method.to_s, delivery_method.to_s, *@args
-        ActionMailer::DeliveryJob.set(options).perform_later(*args)
+        if @obj
+          raise "You've accessed the message before asking to deliver it " \
+            "later, so you may have made local changes that would be " \
+            "silently lost if we enqueued a job to deliver it. Why? Only " \
+            "the mailer method *arguments* are passed with the delivery job! " \
+            "Do not access the message in any way if you mean to deliver it " \
+            "later. Workarounds: 1. don't touch the message before calling " \
+            "#deliver_later, 2. only touch the message *within your mailer " \
+            "method*, or 3. use a custom Active Job instead of #deliver_later."
+        else
+          args = @mailer.name, @mail_method.to_s, delivery_method.to_s, *@args
+          ActionMailer::DeliveryJob.set(options).perform_later(*args)
+        end
       end
   end
 end
diff --git a/actionmailer/lib/rails/generators/mailer/templates/application_mailer.rb b/actionmailer/lib/rails/generators/mailer/templates/application_mailer.rb
index f23e575fe5..00fb9bd48f 100644
--- a/actionmailer/lib/rails/generators/mailer/templates/application_mailer.rb
+++ b/actionmailer/lib/rails/generators/mailer/templates/application_mailer.rb
@@ -1,4 +1,4 @@
-<% module_namespacing do %>
+<% module_namespacing do -%>
 class ApplicationMailer < ActionMailer::Base
   default from: 'from@example.com'
   layout 'mailer'
diff --git a/actionmailer/test/delivery_methods_test.rb b/actionmailer/test/delivery_methods_test.rb
index 2786fe0d07..bcbd036f26 100644
--- a/actionmailer/test/delivery_methods_test.rb
+++ b/actionmailer/test/delivery_methods_test.rb
@@ -39,7 +39,7 @@ class DefaultsDeliveryMethodsTest < ActiveSupport::TestCase
   test "default sendmail settings" do
     settings = {
       location:  '/usr/sbin/sendmail',
-      arguments: '-i -t'
+      arguments: '-i'
     }
     assert_equal settings, ActionMailer::Base.sendmail_settings
   end
diff --git a/actionmailer/test/i18n_with_controller_test.rb b/actionmailer/test/i18n_with_controller_test.rb
index 6124ffeb52..50c4b74eb8 100644
--- a/actionmailer/test/i18n_with_controller_test.rb
+++ b/actionmailer/test/i18n_with_controller_test.rb
@@ -25,7 +25,9 @@ end
 class ActionMailerI18nWithControllerTest < ActionDispatch::IntegrationTest
   Routes = ActionDispatch::Routing::RouteSet.new
   Routes.draw do
-    get ':controller(/:action(/:id))'
+    ActiveSupport::Deprecation.silence do
+      get ':controller(/:action(/:id))'
+    end
   end
 
   class RoutedRackApp
diff --git a/actionmailer/test/message_delivery_test.rb b/actionmailer/test/message_delivery_test.rb
index b834cdd08c..f06d69369f 100644
--- a/actionmailer/test/message_delivery_test.rb
+++ b/actionmailer/test/message_delivery_test.rb
@@ -93,4 +93,12 @@ class MessageDeliveryTest < ActiveSupport::TestCase
       @mail.deliver_later(queue: :another_queue)
     end
   end
+
+  test 'deliver_later after accessing the message is disallowed' do
+    @mail.message # Load the message, which calls the mailer method.
+
+    assert_raise RuntimeError do
+      @mail.deliver_later
+    end
+  end
 end
diff --git a/actionmailer/test/url_test.rb b/actionmailer/test/url_test.rb
index 7928fe9542..70bd05055f 100644
--- a/actionmailer/test/url_test.rb
+++ b/actionmailer/test/url_test.rb
@@ -79,9 +79,11 @@ class ActionMailerUrlTest < ActionMailer::TestCase
     UrlTestMailer.delivery_method = :test
 
     AppRoutes.draw do
-      get ':controller(/:action(/:id))'
-      get '/welcome'  => 'foo#bar', as: 'welcome'
-      get '/dummy_model' => 'foo#baz', as: 'dummy_model'
+      ActiveSupport::Deprecation.silence do
+        get ':controller(/:action(/:id))'
+        get '/welcome'  => 'foo#bar', as: 'welcome'
+        get '/dummy_model' => 'foo#baz', as: 'dummy_model'
+      end
     end
 
     # string
@@ -108,8 +110,10 @@ class ActionMailerUrlTest < ActionMailer::TestCase
     UrlTestMailer.delivery_method = :test
 
     AppRoutes.draw do
-      get ':controller(/:action(/:id))'
-      get '/welcome' => "foo#bar", as: "welcome"
+      ActiveSupport::Deprecation.silence do
+        get ':controller(/:action(/:id))'
+        get '/welcome' => "foo#bar", as: "welcome"
+      end
     end
 
     expected = new_mail