module ActionMailer::Callbacks::ClassMethods

def after_deliver(*filters, &blk)

message's delivery method is finished.
Defines a callback that will get called right after the 
def after_deliver(*filters, &blk)
  set_callback(:deliver, :after, *filters, &blk)
end

def around_deliver(*filters, &blk)

Defines a callback that will get called around the message's deliver method. 
def around_deliver(*filters, &blk)
  set_callback(:deliver, :around, *filters, &blk)
end

def before_deliver(*filters, &blk)

message is sent to the delivery method.
Defines a callback that will get called right before the 
def before_deliver(*filters, &blk)
  set_callback(:deliver, :before, *filters, &blk)
end