Is there not a way to have the Message Screener scan port 25 for the criteria entered in the SMTP Filter when the SMTP Gateway is located on the ISA Server? I use the MSSMTP service as a gateway for my Exchange Server. The SMTP Server is located on the ISA Server machine. The SMTP filter works with no problem if the SMTP Server is not on the ISA Server.
The SMTP filter is an application filter. Application filters do not intercept traffic if they are bound for the local computer. I.e. You cannot have the SMTP filter and SMTP server on the same machine. The SMTP server should be BEHIND the firewall and not ON the firewall.