This problem was seen in other areas via google but no real answer for Mac OS X 10.6. This may apply to Lion/ML too.
If you get an error in the mail.log postfix/smtpd ... status=bounced (mail forwarding loop for email@example.com) and the mail for Mr. Fubar shows in his inbox for a few seconds and then dovecot makes it fly away??
Well I have an answer iCal : Enable Email Invitations! The account you use to set that up gets this "mail forwarding loop" effect. Not sure of the why or how but I did some breif testing and my problems is fixed.
Hope this helps someone down the road.
PS I did not read the iCal Service man yet, I just turned it on. Maybe theres a note in the docs aboout what email address to use, but i doubt it.
Mac Pro, Mac OS X (10.6.8), Server 2 x 2.66Ghz intel Xeon 2006