We're running Exchange 2003 SP2 on three servers in the same site, one of which is a Frontend server, set up to act as a smarthost for the other two, which house the mailboxes.
Everything works great except that, when users request Delivery Receipts from the outside, the receipt ends up in the "messages queued for deferred delivery" queue on one of the mailbox servers. When I look a the email in the queue, it shows "Envelope Recipients: SMTP:email@example.com;". My users do not have SMTP addresses with that domain name, but when I added one to my account and tested, received a receipt message. I realize that adding SMTP addresses like that through a recipient policy is an easy work around, but I'd rather keep it simple and figure out what I'm doing wrong.
It seems like this problem has something to do with the FQDN and Masquerade domain info set in the Advanced Delivery section of the Default SMTP Virtual Servers on the backend.