Can I disagree that Sophos is the best of all virus scanner? 
I tough same thing so I have installed Sophos free in my Company Nethsecurity (NG 6.9 at the moment) in parallel with ClamAV.
ClamAV detects 99% of mail with viruses.
Rarely Sophos and ClamAV detect both the same viruses in mails.
Sophos never detects alone mails with viruses.
You are wrong. Sophos is working alone here and I got already quite some virus alerts.
But in Mailscanner you can choose more than one virus scanner if you want, Amavis plus Sophos for example.
Clamav and sophos work at the same time scanning same emails on my firewall via amavis.
I tough to have better virus detection with both antivirus working together but it wasn’t true.
If a mail contains virus, 99% of the time only clamav detect it, sometime clamav and sophos detect it at same time, but sophos alone never detect a virus in my mails.
Of course you can… I’m sorry to learn about your bad experience on Sophos free tool but… Which is the installation of softwares?
Is Sophos used as SMTP/POP3-IMAP Proxy before the mailserver?
I integrate Sophos with Amavis via Sophie Protocol
Basically i read this howto: https://forums.deeztek.com/viewtopic.php?t=47825# and changed some things (template custom) to adapt to Nethserver NG 6.x.
I don’t tried with Nethserver NG 7.x
Yes, it’s very good. Even more since you have the option to use the Sophos free for Linux version with Mailscanner which is the best of all virus scanners. Plus you have a web gui with the additional app Mailwatch.
This is possible with amavis too as @saitobenkei pointed out.
http://amavis.sourceforge.net/amavis.html#scanners
Upcoming rspamd also has a web interface, what do you think in comparison to mailwatch?
https://community.nethserver.org/t/nethserver-rspamd-a-new-module/8485
I think clamav is a good basic virus scanner and we should discuss if and which scanner is worth implementing additionally to clamav.
the top of rspamd is the history panel, you see each email (only an UID) with the subject/score and why it is accepted, classified as (probably) spam, or rejected