module Mail

def self.register_observer(observer)

which receives the email that is sent.
Your object needs to respond to a single method #delivered_email(mail)

this method.
You can register an object to be informed of every email that is sent through 
def self.register_observer(observer)
  unless @@delivery_notification_observers.include?(observer)
    @@delivery_notification_observers << observer
  end
end