Mailgun delivery method for Ruby Mail gem

.env

MAILGUN_DOMAIN=sandboxwhatevs.mailgun.org
MAILGUN_API_KEY=key-whatevs

Gemfile

gem 'mail'
gem 'mailgun', require: false

initializer.rb

# frozen_string_literal: true
require 'mail/network/delivery_methods/mailgun' # or wherever you put mailgun.rb

Mail.defaults do
  delivery_method Mail::Mailgun,
    domain: ENV.fetch('MAILGUN_DOMAIN'),
    api_key: ENV.fetch('MAILGUN_API_KEY')
end

mailgun.rb

# frozen_string_literal: true
require 'mail/check_delivery_params'
require 'mailgun'

module Mail
  class Mailgun
    include Mail::CheckDeliveryParams

    def initialize(values)
      self.settings = { :api_key => nil, :domain => nil }.merge!(values)
    end

    attr_accessor :settings

    def deliver!(mail)
      message = check_delivery_params(mail)[-1]
      client = ::Mailgun::Client.new settings[:api_key]
      result = client.send_message settings[:domain],
        to: mail.destinations,
        message: message

      settings[:return_response] ? result : self
    end
  end
end