As for my routed hosts - where some customers to use our mail system as a filter and then deliver the scanned and cleaned messages to their MS Exchange server (ick), we have real problems with that because as a router, our system will accept ALL e-mail destined for the final host and our system doesn't reject based on a valid user. Of course, when the system finds out the e-mail address is invalid, it can't return the message because its come from no where or an invalid address.

I'm strongly considering writing a courierfilter that will do the following:

1. For routed domains/hosts, check a database for valid users and reject if the incoming message isn't to a valid user. Make the filter logs the rejection in a way I can parse it from the logs.