class ActionMailer::LogSubscriber

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

def deliver(event)

An email was delivered. 
def deliver(event)
  info do
    perform_deliveries = event.payload[:perform_deliveries]
    if perform_deliveries
      "Delivered mail #{event.payload[:message_id]} (#{event.duration.round(1)}ms)"
    else
      "Skipped delivery of mail #{event.payload[:message_id]} as `perform_deliveries` is false"
    end
  end
  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)
  debug do
    mailer = event.payload[:mailer]
    action = event.payload[:action]
    "#{mailer}##{action}: processed outbound mail in #{event.duration.round(1)}ms"
  end
end

def receive(event)

An email was received. 
def receive(event)
  info { "Received mail (#{event.duration.round(1)}ms)" }
  debug { event.payload[:mail] }
end