Mailer should give self as payload because it contains all the information needed (as the template rendered and the mailer name).

actionmailer/lib/action_mailer/base.rb

@@ -499,7 +499,7 @@ module ActionMailer #:nodoc:
         logger.debug "\n#{mail.encoded}"
       end
 
-      ActiveSupport::Notifications.instrument("action_mailer.deliver", :mail => mail) do
+      ActiveSupport::Notifications.instrument("action_mailer.deliver", :mail => self) do
         begin
           self.delivery_method.perform_delivery(mail) if perform_deliveries
         rescue Exception => e # Net::SMTP errors or sendmail pipe errors