A very-much-in-need-of-improvement way to replace DelayedJob with SuckerPunch for your application mailers.

delayable_mailer.rb

class DelayableMailer < ActionMailer::Base
  class QueueSetupError < StandardError; end

  class << self
    def queue(options)
      @queue_name = options.fetch(:name)
      @workers    = options.fetch(:workers) { 2 }
      setup_queue
    end

    def delay
      if !setup?
        raise QueueSetupError, "please setup a queue before using a Delayable"
      end

      DelayedMethod.new(@queue_name)
    end

    def setup?
      !!@setup
    end

    def setup_queue
      scope  = self
      worker = Class.new do
        include SuckerPunch::Worker

        define_method(:perform) do |method_name, *args|
          ActiveRecord::Base.connection_pool.with_connection do
            scope.public_send(method_name, *args).deliver
          end
        end
      end

      SuckerPunch.queue name: @queue_name, worker: worker, workers: @workers
      @setup = true
    end
  end

  class DelayedMethod
    attr_reader :queue_name
    private :queue_name

    def initialize(queue_name)
      @queue_name = queue_name
    end

    def method_missing(method_name, *args)
      SuckerPunch::Queue[queue_name].async.perform(method_name.to_sym, *args)
    end
  end
end

class SampleMailer < DelayableMailer
  queue :sample_mailer_queue
  
  def sample_message(user_id)
    # ...
  end
end

user = User.find(1)
# ...
SampleMailer.delay.sample_message(user.id)