I have a client with a zentyal server who uses it purely as a mail server.
The Zentyal box has a single NIC
Is connected to the switch behind a Sophos UTM (with TCP 443 forwarded)
Incoming mail is using Fetchmail from POP accounts
Clients are all local Outlook 2016 with 1 or 2 iphones, all using Activesync
SOGo is in place for shared calendars and a webmail portal
Greylist and spam filter as well as a file type filter for attachments and ClamAV
Can this scenario be reproduced on Nethserver?
With a single NIC and fetchmail rather than changing the MX records?
With greylisting and the other security features?