Why OpenBSD? PF tools: spamd

Minimally SMTP-capable daemon, divert-to hook-in (used to be rdr-to)

Initially a blacklist-importing tarpit (answer 1 byte per packet, 1 packet per second or less)

Since OpenBSD 4.1, greylisting by default

OpenBSD 3.8 introduced greytrapping: use bogus addresses in own domains as spammer traps

Combines nicely with content filtering (and reduces incoming volume to be content filtered)

I've supplied spam-eating appliances based on this (insert in SMTP signal path, leave others untouched)

See also http://bsdly.blogspot.com keyword spam