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