Messagelabs email filtering
After years of trying everything on our in-house email server, including the TMDA challenge-response system, and having suffered countless DOS attacks and anything up to 10k spams/day, we have finally capitulated and signed up to Messagelabs for £420/year.
This does up to 10 users (distinct user mailboxes) with no apparent limit on the # of domains which receive email, and no apparent limit on the # of aliases on each domain.
It works amazingly well.
Their claims for false positives (real emails dumped) are astonishing at 1/300k or better, which is why I went for it, because we have customers in countries which are routinely blacklisted by the standard blacklists.
The config of the service is really clunky though... but once set up, it is fine and adding/removing users is really easy.