I am the SysAdmin for a large ISP here in the UK and we use Vexira Mailarmor together with Sendmail and Milter We have about 500 concurrent connections all day long and we tried many different antivirus vendors but choseVexira because it could handle this amount of users without a problem.
What do I like best about Vexira?
Unpacks and extracts all MIME encoded messages without needing any other helper apps
Automatic updating
Very fast virus scanning
Reasonable price
We tested and evaluated vexira for about a month and it was clear that this app is a awesome antivirus.