> 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).

I forgot to mention (since you asked for docs). I wrote a chapter about 
spamd for The OpenBSD PF Packet Filter Book (now available at 
www.Amazon.co.uk). Feel free to email me for the chapter if interested.