DNS Blacklist Packet Filter is a BSD/Linux netfilter client that decides whether to accept or drop +packets based on the results of a DNS blacklist query (such as MAPS, SORBS, or SPEWS, to name a few). +One use is to filter all incoming SMTP SYN packets for spam filtering.
Semi's Spam Milter (sspamm) is a spam filter for Sendmail that utilizes spambayes, heurestic spam +filtering, and virus scanning (BitDefender, not included).