I believe you could forget about free antivirus options. Free antivirus beside clamav is usually for private/personal use only and usually includes software to be used on personal desktop computers and not on servers or to be invoked by CLI or API calls. However, licenses for "appliance" usage is often different, so there may be attractive options. Recent versions of PMG already e.g. included Avira, which I believe is one of the best options, but with open-sourcing PMG, Avira is not included any more. Maybe in future subscriptions it will get an option again (hopefully). Beside Avira I would like to see Sophos, however Sophos is already used for endpoint protection, so it would make sense to use another vendor. Avast had bad publicity in the past, I would not prefer to use that, however, it's the only alternative available currently, also needs to be licensed.