How do I create a Mailer Observer

I’m surprised how little there is in Rails’ documentation about this.

Basically, ActionMailer in Rails 3 introduces the use of Interceptors (called before the message is sent) and Observers (after the message is sent).

To set up an Observer, add the following to an initializer:

class MailObserver
  def self.delivered_email(message)
    # Do whatever you want with the message in here
  end
end

ActionMailer::Base.register_observer(MailObserver)

Now, the delivered_email method will run every time your app sends an e-mail. However, you will only have access to the actual Mail message.

To register an Interceptor instead, do the same as above, replacing register_observer with register_interceptor, and renaming the method from self.delivered_email to self.delivering_email.

This Railscast was the best source I could find for info on this (they only talk about interceptors, but the concept is the same for observers).