Initial version - the idea is to make Mail::QueuedDelivery a proxy that takes original delivery_method as a parameter

delayed_job_mailer.rb

# lib/mail/queued_delivery.rb
require 'mail'

module Delayed
  class PerformableMail
    attr_reader :mail

    def initialize(mail)
      @mail = mail
    end

    def display_name
      "#{mail.class}:#{mail.object_id}"
    end

    def perform
      mail.delivery_method.deliver!(mail)
    end
  end
end

module Mail
  class QueuedDelivery < Sendmail
    def deliver!(mail)
      # Switch email delivery method to a non-queued version,
      # but keep the custom settings if present
      mail.delivery_method(:sendmail)
      Delayed::Job.enqueue Delayed::PerformableMail.new(mail)
      self
    end
  end
end

# config/application.rb
require Rails.root.join("lib/mail/queued_delivery")
ActionMailer::Base.add_delivery_method :queued, Mail::QueuedDelivery

# config/environments/production.rb
config.action_mailer.delivery_method = :queued