Threaded mail delivery in rails

async_smtp_delivery_method.rb

# lib/async_smtp_delivery_method.rb
require 'mail'

class AsyncSmtpDeliveryMethod

  def initialize(settings)
    @settings = settings
  end

  def deliver!(mail)
    Thread.start do
      begin
        Mail::SMTP.new(@settings).deliver!(mail)
      rescue Exception => ex
        ::Rails.logger.error "Failed to send email: #{ex.inspect}"
        raise
      end
    end
  end

end

ActionMailer::Base.add_delivery_method :async_smtp, AsyncSmtpDeliveryMethod 

# KEPP IN MIND:
# It will only run as long as process does. So to test your email form rails runner add a wait period at exit, like this:
# rails runner "UserMailer.welcome.deliver; at_exit { sleep 15 }"

production.rb

# environments/production.rb
require 'async_smtp_delivery_method'

YourApp::Application.configure do
  config.action_mailer.delivery_method = :async_smtp
  config.action_mailer.async_smtp_settings = {
    :address              => AppConfig.smtp.address,
    :port                 => AppConfig.smtp.port,
    :user_name            => AppConfig.smtp.user_name,
    :password             => AppConfig.smtp.password,
    :authentication       => :plain,
    :enable_starttls_auto => true
  }
end