The easiest way IMO is to create a non-login account for the user(s):
useradd -g joe -s /sbin/nologin -d /dev/null -c 'Joe Bloggs' joe
This creates a Linux account for the user joe
with a nologin shell, and the home directory is black hole.
That's probably the easiest way.
By default his mail will end up in /var/spool/mail/joe
until fetched via pop, http, or imap etc.
Don't let Linux screw your head up, take it slow and don't get frustrated, once you learn enough you'll love it.