Now I think that policy and or after queue filtering are good solutions, but both seem rather complex for a relatively easy problem.
If you will create something what policyd does (www.policyd.org) using http://www.postfix.org/SMTPD_POLICY_README.html, you are able to do something with the mail except the address rewriting, so you cannot forward the mail anywhere. So this is IMHO not a good way for you.
What about some kind of milter? If you write a milter rewriting address, you can forward the mail to an alias that sends the mail into some pipe program acting as autoreply...
Tomas
