class ActionMailer::LogSubscriber

email is delivered and received.
Implements the ActiveSupport::LogSubscriber for logging notifications when

def deliver(event)

An email was delivered. 
def deliver(event)
  return unless logger.info?
  recipients = Array(event.payload[:to]).join(', ')
  info("\nSent mail to #{recipients} (#{event.duration.round(1)}ms)")
  debug(event.payload[:mail])
end

def logger

Use the logger configured for ActionMailer::Base 
def logger
  ActionMailer::Base.logger
end

def process(event)

An email was generated. 
def process(event)
  mailer = event.payload[:mailer]
  action = event.payload[:action]
  debug("\n#{mailer}##{action}: processed outbound mail in #{event.duration.round(1)}ms")
end

def receive(event)

An email was received. 
def receive(event)
  return unless logger.info?
  info("\nReceived mail (#{event.duration.round(1)}ms)")
  debug(event.payload[:mail])
end