Another alternative is to use pkgsrc/mail/spamd. I use it with PF (and I 
am sure it could be modified to work with ipf too).

It is not integrated with the MTA though. But in combination with the 
spamd-setup it can use custom blacklists too.

Also, spamd greylist solution delays mails to third acceptable attempt 
(where others often will work on second).