I have Qmail on Linux with spamassassin and qmail-scanner (Sophos) as the high-priority MX record for all the domains we host. The SMTPROUTES file in Qmail then sends the clean traffic out the backdoor nic to my SMTP servers. Since this runs on a hand-me-down old Windows server converted to Linux... and since the software is free... this is a simple, cheap, and very effective solution to SMTP pre-processing.
: Has anyone come across a front-end Spam filter for the IIS SMTP Server? That is, something that will filter incoming mail and selectively block unwanted mail before transmission, preferrably returning some sort of SMTP error such as "Relaying Denied" (or similar) to the sending server.