Better way to create the crazy email Mailgun wants you to send for batch emails through SMTP.

action_mailer.rb

ActionMailer::Base.register_interceptor(MailgunBatch)

mailgun_batch.rb

class MailgunBatch

  # This is where the magic happens
  def self.delivering_email(message)
    if batch?(message)
      add_recipient_variables(message)
    end
  end

  # Have either of the batch headers been set?
  def self.batch?(message)
    message.header_fields.any? { |header_field| header_field.name.to_s.starts_with?('X-Mailgun-Batch') }
  end

  def self.add_recipient_variables(message)

    # Building the individual recipient email
    recipient_message = Mail.new(message)
    recipient_message.to = '%recipient%'
    recipient_variables = retrive_recipient_variables(recipient_message)
    recipient_variables.reverse_merge! extract_to_addresses(message)

    # Mail gun headers should only be on the main message
    remove_headers_starting_with!(recipient_message, 'X-Mailgun')

    message.body = nil # Clearing the body so we can build the new message
    message.content_type = 'multipart/mailgun-variables' # Setting a special content type

    # The recipient variables hash is converted to base64 encoded json then added to the message
    message.add_part Mail::Part.new(content_type: 'application/json', content_transfer_encoding: 'base64', body: Base64.encode64(recipient_variables.to_json))

    #Removing the batch headers after already addeding the encoded json
    remove_headers_starting_with!(message, 'X-Mailgun-Batch')

    # Adding the customized message with headers
    message.add_part Mail::Part.new(content_type: 'message/rfc822', body: recipient_message.encoded)

  end

  # Takes headers that are either json or hashes and merges them together into one hash
  def self.retrive_recipient_variables(message)
    message.header_fields.each_with_object({}) do |field, h|
      if field.name === 'X-Mailgun-Batch-Variables'
        case field.value
        when Hash then h.merge! field.value
        when String then h.merge! JSON.parse(field.value)
        end
      end
    end
  end

  # Pulls out the to addresses and adds them as empty recipient variables
  # Enable using headers['X-Mailgun-Batch-Send'] = true
  def self.extract_to_addresses(message)
    message.to.each.each_with_object({}) do |address, h|
      h[address] = {}
    end
  end

  # Delete any headers that start with the given string
  def self.remove_headers_starting_with!(message, str)
    message.header.fields.delete_if { |header| header.name.to_s.starts_with?(str) }
  end
end