Because I installed Linux Mint on my old patato PC, I searched for AntiVirus available for Linux. From what I read, ClamAv is mostly used as just another (free) layer for mail servers
Most website/mail servers run a Linux OS. ClamAV is used with other protection layers like: Firewall + Snort + SPAM assassin + ClamAV + Yara. Combined they should keep out most nasties, before reaching corporate (Windows OS dominated) networks. As far as I understand, it makes sense to use a different OS to protect your main infrastructure, since most attacks are oriented against the windows environment, it is 'safer' to analyse and deconstruct malwares trying to infect your infrastructure. In corporate networks it is just the second layer after the DNS. The third layer is often delivered by commercial security services.
In Linux forums ex-Windows users seeking for an AV and help to use ClamAv in realtime/on demand are often mocked at. Most Linux users seem to use passive AV protection (so I also decided to go with the flow and drop the Windows-urge for AV-protection).