... will only make the spammers switch to a distrubuted attack method using trojaned virus hosts sending 1 mail message at a time.
"switch to"? i don't know where you're getting your spam from, but the spammers "switched to" that methodology a long long time ago.
The evolution of spam/viruses is astounding and getting quicker all the time.
which is why many of us have fought so hard against bad solutions. the baynesian filtering crowd is the worst. it's like treating every illness with antibiotics... what you end up with is a lot MORE illness in the medium to longer term, due to antibiotic-immune mutations.
... [OpenBSD] Spamd is really nice for hurting spammers and/or relays where it can... in their spool.
spammers don't have spools. they don't care if you get their stuff or not, and they don't retry. that's why greylisting has been so effective -- to combat it the spammers would have to add the one thing they cannot afford: "state." see http://www.rhyolite.com/dcc/ for how to get started. -- Paul Vixie