virus software for Exchange server

TrendMicro's ScanMail for Exchange, is one of the best out there. About $20/mailbox.
Sybari's AntiGen is the best at about $25/mailbox.
Personally use ScanMail and works great, causes very little performance issues, is very configurable (has a feature to block certain attachments by name or extension), and Trend has been very good about DAT updates.
AntiGen is even a little better on performance, and all the same features, but a little harder to find through resellers.

The Best one on the market for Exchange 5.5 And 2000 is MacAfee GroupSheild and NetShield which comes with total Exchange Support.
I rate this product very highly

The company I work for uses TrendMicro's scanmail

Any virus scan software that uses an AVAPI process to scan. Trend Micro's ScanMail has this capability.
AVAPI scanning is good because it scans before the messages are committed to the information store.
Traditional MAPI based scan solutions can only scan the mailboxes after the message has been committed to the IS. This creates problems when messages are still being sent and received and sitting in the transaction logs.

I have a pretty complete list of what's available at http://www.slipstick.com/addins/antivirus.htm#server. I know some people don't like the AVAPI approach, maybe because of the load on the server. Sybari has their own interface that they've hacked into the information store, pre-AVAPI.

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