Your quick responses are appreciated!
I've installed nethserver-roundcube mail via yum (hope that was OK) which installed 28 new pkgs and upgraded 5. Than I was able to log into webmail as the user I had created! Sent myself a test email from there, but have yet to receive it.
Meanwhile, the point of this whole exercise was to allow a client to read the 4.5 GB of email from the Zentyal 3.2 server that developed segfaults, for unknown reason (in an instance of virtualbox). Ethernet even stopped working, though the interface is properly configured. When moving the contents of /var/vmail/domainname/* to a fresh copy of Zentyal 3.5, configured for same virtual domain, the old mail was invisible. That seems to be due to the use of ldap, which I haven't a clue how to migrate, if it is even possible. Yet I think ldap is only used as an optional feature. So, I've been searching for a distro using postfix without ldap, thinking then the emails would be accessible again.
The NethServer postfix does not use /var/vmail, and I'm still searching for the place where old emails could be untarred.