Seriously... How hard is it to setup the MX boxen to only allow 4 email addresses to pass for that particular domain, rejecting all others in the SMTP conversation?
Unless the customer is dropping BIG DADDY $$$ with you, tell him policy change and that he isn't losing any email if you do not do a catchall for his domain That postmaster thing is a monster. Send the postmaster stuff to that customer and see how soon they want it turned off ;-> Otherwise do what Kris said and push or pull or whatever all the validrcptto's out to the MX's - rh
