I'd recommend a Debian Stable or Testing based Exim mailserver, coupled with vexim to provide mail service to multiple domains on one server. For AV, use either ClamAV or a commercial scanner.

I don't do spam filtering/greylisting on the server side, so I can't help you there.

I've been itching to try out RoundCube (http://roundcube.net/) for a few years now but haven't found the copious free time to check it out. It looks really pretty and the feature list looks nice.

Switched from SquirrelMail to RoundCube a few years ago and am happy with it. It's no Gmail interface, though. :-(