And welcome to the NethServer Forum!
Are you, by any chance, from Lucerne in Switzerland?
(See my “Tag” on the top left of any of my posts…)
Email, is something invented in the 70ies for the Internet - and it’s still one of the major use protocolls for communication today. Unfortunately, spam is a large part of this, but that’s another story!
In any case, mail was designed for the Internet, and as such, it needs (AFAIK) a domainname after the “@”.
It wasn’t designed for Windows systems, which often only use the hostname, limited to 15 CHRs…
AFAIK, NethServer won’t accept a Domain without a dot in it.
My 2 cents