Are you filtering from Exchange or IIS?
The link that I sent you shows that building an event sink is not for the faint of heart. If you're familiar with VBScript, you should be able to start your solution from there.
But really, I stand by my comments above about using the Anti-Virus / Anti-Spam software to do this type of event handling.
PSC
Governments and corporations need people like you and me. We are samurai. The keyboard cowboys. And all those other people out there who have no idea what's going on are the cattle. Mooo! --Mr. The Plague, from the movie "Hackers