set Message-ID header in ActionMailer?

I'm trying to set my own Message-ID when sending mail... not having
any luck. It seems that my ID header is overwritten. i've tried both
message_id and headers below, nothing seems to work:

def setup_email(message, person)
  @recipients = "#{person.email}"
  @from = "myapp@example.com"
  @message_id = "my-custom-message-id"
  @headers = {'Message-ID' => message.smtp_message_identifier}
  @sent_on = Time.now
  @body[:msg] = message.body
end

any suggestions?