Without knowning more details (what server- and what firewall-arch) I would respond to
I want to avoid viruses on the server´s system files and in E-Mails, sent and received over the server
Scan e-Mails on their way to the server via a mailgateway. It wouldn't be the first time there was some strange side effect when doing the scanning on the same system having the normal mail service on. Get them out before they reach the final destination server (and the user) and run an additional AV on the normal filesystem of the server for file services.
But I would not run that kind of thing on the firewall itself. Keep the firewall architecture as clean as possible and don't mix it with further services if they don't have necessarily to do with it. E.g. split the fw-arch into three nets, WAN, DMZ and LAN and setup the mailgateway in the DMZ area. So you don't have any probably "bad things" in your LAN before it passes all your desired tests.
Just my thoughts on this :)