Index: app/models/mailer.rb
===================================================================
--- app/models/mailer.rb	(revision 10015)
+++ app/models/mailer.rb	(working copy)
@@ -418,6 +418,12 @@
   
   def self.deliver_mail(mail)
     return false if mail.to.blank? && mail.cc.blank? && mail.bcc.blank?
+    if self.async
+      Thread.start do
+        super
+      end
+      return true
+    end
     super
   end
 
Index: config/initializers/10-patches.rb
===================================================================
--- config/initializers/10-patches.rb	(revision 9976)
+++ config/initializers/10-patches.rb	(working copy)
@@ -47,9 +47,16 @@
 
 require 'mail'
 
+module ActionMailer
+  class Base
+    cattr_accessor :async
+  end
+end
+
 module DeliveryMethods
   class AsyncSMTP < ::Mail::SMTP
     def deliver!(*args)
+      ActiveSupport::Deprecation.warn "async_smtp delivery method is deprecated and will be removed in Redmine 2.2. Use the async property at email_delivery level instead."
       Thread.start do
         super *args
       end
@@ -58,6 +65,7 @@
 
   class AsyncSendmail < ::Mail::Sendmail
     def deliver!(*args)
+      ActiveSupport::Deprecation.warn "async_sendmail delivery method is deprecated and will be removed in Redmine 2.2. Use the async property at email_delivery level instead."
       Thread.start do
         super *args
       end