MessageWall is an SMTP proxy. It sits between the outside world and
your mail server and keeps out viruses, spam, and mail relaying.
It offers filtering via header and body checks, DNS-based blacklists
for IP addresses (DNSBL) and domains (RHSBL), DNS-based distributed
checksumming (DNS DCC) similar to Vipul's Razor, and Open AntiVirus
pattern scanning.  ke many existing ISP-level filtering solutions,
it offers customization of filtering features on a per-address
basis. Unlike SpamAssassin, it communicates via SMTP, so it works
with any MTA. It also supports scoring based on rules with a rejection
threshold; this allows hybrid warning/rejection configurations based
on how many and which rules a message matches.