Based on the headers you provided, this message does appear to be a Message Disposition Notification (MDN) - specifically a read receipt. It also looks like this message was originally generated by an Outlook client. I did a quick test and was able to replicate this behavior using an IMAP account in Outlook. Basically, it looks like Outlook is submitting the MDN message to SmarterMail over SMTP. As part of the submission, Outlook is providing the empty Return-Path, which SmarterMail uses when sending the message to the remote server.
Andrew Barker
Software Developer
SmarterTools Inc.
(877) 357-6278
www.smartertools.com