Vautrin makes a very good point above. The only people likely to be writing to addresses you don't support aren't the sort of people who have their headers set up so they can properly receieve bounces.
I do something similar to you although the details will be different.
I have a hosting account that forwards all mail to the domain to a given address. fetchmail then retrieves the mail, and procmail filters it. Anything to a 'whitelist' address gets filtered by SpamAssassin and kept. Anything to a blacklist gets permanently deleted. Anything not on a list gets logged and held in quarantine, a report then comes to me telling me that 'A message addresses to something@domain.com' has been receieved from 'someone@somewhere.com''. I can then add that address to either list and the quarantine releases the mail. (SpamAssassin itself uses whitelists so that friends can send me stuff without it getting munged!)
The reason I outline all this is so that I can tell you that I never bounce a message. I figure a bounce is yet another email going nowhere that noone will read.
(The reason I do all this rather than just deleting stuff to unrecognised addresses is that I make up addresses whenever I need to. If I register here with 'perlmonks@mydomain.com' rather than 'me@mydomain.com' then I can firstly create easy-to-use filters and secondly, if I get spam sent to that address I know the owners of this site sold me out!) | 