Getting Mailgun SMTP Batch emails to work in ActionMailer.

mailgun_batch_smtp.rb

class FoobarMailer < BaseMailer
  # The method you'll actually call to generate the batched email
  def batched_foobar
    # Recipient Variables
    recipients = {"you@somewhere.com" => {"name" => "You Youington"}}

    mail(to: "me@somewhere.com") do |format|
      # Mailgun requires you to base64 your Recipient Variables JSON
      format.custom('application/json', content_transfer_encoding: "base64") do
        render text: Base64.encode64(recipients.to_json)
      end
      # The standalone email, including both headers and body
      format.custom('message/rfc822') do
        render text: FoobarMailer.regular_foobar.to_s
      end
    end
  end

  # The standalone email... used to generate one part of the batched email
  def regular_foobar()
    mail(to: "%recipient%") do |format|
    	format.text { render text: "Hello %recipient.name%" }
    end
  end
end

# Example
FoobarMailer.batched_foobar.deliver