If you have two nodes / sites, it would be easily possible to automate…
On the second site, set up an echo mail account, which is limited to senders from site1 (Too much spam otherwise!). This second site could be at home, at a friends server, anywhere. It just needs to return mail (Using a Mailfilter!).
An echo server will “echo” the mail back to the sender. These were very popular in the 1990s to test mail. Any better provider had one.
On your server, have POP3, IMAP and also SMTP monitored with Zabbix.
You may also want to monitor the MariaDB used by Webtop… Zabbix can do that…
Using Crontab (or whatever you prefer), send a regular mail to the echo server @Testtime
Use a local job to daily verify the maillog on your NethServer and search for that specific mail coming back in, usually 1-2 minutes after you sent it.
This check can be returned to Zabbix and you got a daily, automated test which actually sends and recieve mail and verify’s it.
My 2 cents