Hi John,
I'm not sure it's a good idea to use an ACTIVE antivirus on a smartermail server, at least if it actively scans every opened file, mainly because if there is a virus in a mail your antivirus will quarantine a whole day of mails (mails are saved in daily files).
So one unique infected mail could potentially remove a whole day of mail content for your customer
I think though there are Windows defender tweaks when you install smartermail so it doesn't cause this, but not sure it's the case for other antiviruses, in fact 99% sure it isn't.
Sébastien Riccio
System & Network Admin
https://swisscenter.com