What I want to do is receive mail from our external server that filters spam and reject mail from all other locations. Is this possible to do with Exchange filters? We use Exchange 2003. The spam got so bad that it was slowing our other Internet activities so we now have an external server for spam filtering. About 85% of our mail is rejected as spam before it reaches our firewall and it is very accurate at identifying spam. All the valid mail is sent from the one external server to our exchange server. There has been a small but increasingly annoying amount of mail that does not use our published mx record but instead sends the mail directly to our exchange server. I want to prevent that from happening.

Configure your firewall to block incoming traffic on port 25 with the exception to your external servers.