I have NDRs for *legitimate* outbound email from extra addresses that were not on the original message created by the user.
For example, user A sends email to user C@work.com about something. A little later user A will get an NDR saying "Could not deliver your message to email@example.com. A copy of the message is below" and it's the legit email.
Can you offer me some troubleshooting and maintenance things to try? I'm an exchange admin who fell into Qmail admin almost by accident, so I don't really know what I'm doing. I know my basic way around the unix command line, but not too deep.
Thanks for your time.