OpenBSD has some really good anti spam software, namely spamd - spam deferral daemon. Not to be confused with spamd from spamassassin. One useful piece of software created by Daniel Hartmeier is relaydb which you can read more about at Annoying spammers.
As nice as OpenBSD is as an operating system it's sometimes useful to harvest spammers IPs on a linux machine, then feed relaydb with your spam emails. Then you can copy across the harvested IPs over to your OpenBSD machine running your firewall.
With the help of others I've created a patch so that relaydb will build on a linux machine. I've done this on a Debian Sarge machine. Also I required the Debian package libdb4.1-dev (Version: 4.1.25-18). There is a small bug in the dbopen code line 561 on the patched version. I'd welcome any patches to fix this small bug.