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
I'm strongly considering writing a courierfilter that will do the
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.
I haven't tried it myself, but I have been told that Exchange can act
as an LDAP server. So you can use LDAP to verify users for your