Hard to tell. The e-mails I get back by POP3 all have valid return paths, in case of googlemail both the empty one and a valid one, in case of our own mail server just a valid one. So I can't tell if that's being added by the server/your component or if it's already there when receiving the mail on the server :-(
That's exactly the same result as when just adding a return path header myself and using smtp.Send(mail).
Any way to log what's actually being sent in .eml form or something?
Edit: I checked on our mail server, and the valid return path is there, too, so it's not due to POP3 downloading. So question is if it's being correctly sent and the return path is added by the mail server, or if a valid return path is being sent.