MAIL FORWARDING
       For the sake of reliability, forwarded mail is  re-submit-
       ted  as  a new message, so that each recipient has a sepa-
       rate on-file delivery status record.

       In order to stop mail forwarding loops early, the software
       adds an optional Delivered-To: header with the final enve-
       lope recipient address. If mail arrives  for  a  recipient
       that is already listed in a Delivered-To: header, the mes-
       sage is bounced.