delayed_mailer.rb

# Queue all email with delayed job automagically.

module DelayedMailer
  extend ActiveSupport::Concern

  included do
    alias_method_chain :deliver, :delayed_job
  end

  class MailJob < Struct.new(:message)
    def perform
      message.deliver_without_delayed_job
    end
  end

  def deliver_with_delayed_job
    Delayed::Job.enqueue(MailJob.new(self))
    self
  end
end

Mail::Message.send(:include, DelayedMailer)

class ActionMailer::Base
  def self.delay(*args)
    raise "already queuing your email"
  end
end