>Spam filtering appliances and services sometimes run as, effectively, >proxies rather than store-and-forward MTAs. Postini works in this >way, I think, as do some of the enterprise filtering appliances that >people protect their Exchange servers with.
Yes. Postini even has a patent on the technique of using callforwards to provide realtime responses in a spam filtering system, although I suspect that if they ever litigated it, they'd find it had been invented multiple times long before they did. Don't Barracudas also work that way? R's, John
