My unequivocal recommendation: Postini. As an administrator for thousands of users and end user since 2002. Postini has been and is rock solid, with zero false positives, no hardware or software to manage, I can lockdown SMTP at the firewall (no anonymous SMTP connections permitted; enhances security and decreases exposure to Exchange/SMTP vulnerabilities), easy to use (no detailed training required for end users), all at great price!
I have tried appliances, open source solutions, software on an MTA or even Exchange (smaller sites), and they all were a waste of time and users were always complaining about false positives and/or too much spam. Postini allows me to concentrate on my job (make all of IT work/prevent things from breaking, provide users the ability to be more productive, ensure secure operations/enforce policies, etc.)
P.S. Email me off list if you want a trial.
I am starting to look for a anti-spam product and was wondering what is being used by the list members.
My soon to be environment will be a single Exchange 2003 server, front ended by a ISA 2006 server.
All-Mode Communications, Inc.
1725 Dryden Road
Freeville, New York 13068
(607) 347-4164 x440
1-877-ALLMODE (toll free)