Guide update for Async ActionMailer

guides/source/action_mailer_basics.textile

@@ -457,6 +457,7 @@ The following configuration options are best made in one of the environment file
 |+delivery_method+|Defines a delivery method. Possible values are <tt>:smtp</tt> (default), <tt>:sendmail</tt>, <tt>:file</tt> and <tt>:test</tt>.|
 |+perform_deliveries+|Determines whether deliveries are actually carried out when the +deliver+ method is invoked on the Mail message. By default they are, but this can be turned off to help functional testing.|
 |+deliveries+|Keeps an array of all the emails sent out through the Action Mailer with delivery_method :test. Most useful for unit and functional testing.|
+|+async+|Setting this flag will turn on asynchronous message sending, message rendering and delivery will be pushed to <tt>Rails.queue</tt> for processing.|
 
 h4. Example Action Mailer Configuration
 
@@ -514,3 +515,33 @@ end
 </ruby>
 
 In the test we send the email and store the returned object in the +email+ variable. We then ensure that it was sent (the first assert), then, in the second batch of assertions, we ensure that the email does indeed contain what we expect.
+
+h3. Asynchronous
+
+You can turn on application-wide asynchronous message sending by adding to your <tt>config/application.rb</tt> file:
+
+<ruby>
+config.action_mailer.async = true
+</ruby>
+
+Alternatively you can turn on async within specific mailers:
+
+<ruby>
+class WelcomeMailer < ActionMailer::Base
+  self.async = true
+end
+</ruby>
+
+h4. Custom Queues
+
+If you need a different queue than <tt>Rails.queue</tt> for your mailer you can override <tt>ActionMailer::Base#queue</tt>:
+
+<ruby>
+class WelcomeMailer < ActionMailer::Base
+  def queue
+    MyQueue.new
+  end
+end
+</ruby>
+
+Your custom queue should expect a job that responds to <tt>#run</tt>.