rails--rails/actionmailer/lib/action_mailer/async.rb

require 'delegate'

module ActionMailer
  module Async
    def method_missing(method_name, *args)
      if action_methods.include?(method_name.to_s)
        QueuedMessage.new(queue, self, method_name, *args)
      else
        super
      end
    end

    def queue
      Rails.queue
    end

    class QueuedMessage < ::Delegator
      attr_reader :queue

      def initialize(queue, mailer_class, method_name, *args)
        @queue        = queue
        @mailer_class = mailer_class
        @method_name  = method_name
        @args         = args
      end

      def __getobj__
        @actual_message ||= @mailer_class.send(:new, @method_name, *@args).message
      end

      def run
        __getobj__.deliver
      end

      # Will push the message onto the Queue to be processed
      def deliver
        @queue << self
      end
    end
  end
end