Email Filtering Software/Vendors/ASPs/Resources

AV software vendors (server- and client-based)
UNIX
*Clam AntiVirus http://clamav.elektrapro.com/
*Open AntiVirus http://www.openantivirus.org/
RAV http://www.ravantivirus.com/
Sophos http://www.sophos.com/
Kaspersky http://www.kaspersky.com/
(& many other commercial vendors)

Windows
Symantec http://www.symantec.com/
RAV http://www.ravantivirus.com/
Kaspersky http://www.kaspersky.com/
Sophos http://www.sophos.com/
(& many other commercial vendors)
AS Techniques and sample software/services
IP-BASED CHECKS
centralized/authoritative: MAPS, ORDB, Osirusoft, SPEWS, SpamCop, Spamhaus
distributed/trust-based: Trustic
self-administered: rules in most firewalls and MTAs

PATTERN MATCHING
header patterns: SpamAssassin, Habeas
body patterns: SpamAssassin

DISTRIBUTED WHOLE-MESSAGE CHECKSUMS
Vipul's Razor/Cloudmark, DCC

STATISTICAL COMPARISON TO KNOWN GOOD/BAD EMAIL
Spammunition, SpamAssassin 2.5, Mail (Mac OS X)

Server-based AS software
UNIX
*SpamAssassin http://spamassassin.org/
*MailScanner http://www.mailscanner.info/
*Vipul's Razor (Cloudmark's open-source predecessor) http://razor.sourceforge.net/
*DCC http://www.rhyolite.com/anti-spam/dcc/
Message Wall http://www.messagewall.org/
PureMessage http://www.activestate.com/Products/PureMessage/
BrightMail http://www.brightmail.com/

Windows
SpamAssassin Pro (for Exchange) http://www.deersoft.com/
Cloudmark Authority http://www.cloudmark.com/
BrightMail http://www.brightmail.com/
DynaComm iMail http://www.dciseries.com/products/imail/
client-based AS software
Windows
*Spammunition (for Outlook) http://www.upserve.com/spammunition/default.asp
*JoeEmail http://joeemail.sourceforge.net/
SpamButcher http://www.spambutcher.com/
Spam Bully (for Outlook / Outlook Express) http://www.spambully.com/
Cloudmark SpamNet (for Outlook) http://www.cloudmark.com/
SpamAssassin Pro (for Outlook) http://www.deersoft.com/
Spamnix (for Eudora) http://www.spamnix.com/

Mac OS 9/X
*Mail (included in Mac OS X 10.2) http://www.apple.com/macosx/jaguar/mail.html
Spamfire http://www.matterform.com/
hosted (ASP) AV and AS services
Postini http://www.postini.com/
StarSure CleanMail http://www.star.net.uk/products/security_cleanmail.asp
SpamCop Corporate Email Services http://mail.spamcop.net/corporate.php
ipop.com http://www.ipop.com/Solutions/EmailFiltering/Default.asp
Electric Embers NPOshield http://electricembers.net/
Electric Embers NPOMail http://electricembers.net/

blacklist/whitelist services
DNS blacklists
MAPS (RBL, DUL, RSS, NML) http://mail-abuse.org/
ORDB http://www.ordb.org/
Osirusoft http://relays.osirusoft.com/
SPEWS http://www.spews.org/
SpamCop http://spamcop.net/
Spamhaus (SBL) http://www.spamhaus.org/
Trustic http://www.trustic.com/

Whitelist warrants
Habeas http://habeas.com/
Bonded Sender Program http://www.bondedsender.org/
Starting points for further reading
http://spamconference.org/proceedings2003.html - proceedings from the 2003 Spam Conference held at MIT, the cutting edge of AS research

http://paulgraham.com/spam.html - the paper that introduced Bayesian statistical analysis to spam filtering

http://www.scotsnewsletter.com/best_of/fightspam.htm - a collections of Let's Fight Spam articles from Scot's Newsleter

http://theory.whirlycott.com/~phil/antispam/rbl-bad/rbl-bad.html - an argument against using blacklists for AS

* Free/open-source, free (re: $$), or included with OS