Your Dovecot service does not start.
/var/log/dovecot.log for entries like this:
Sep 16 09:53:11 ucs dovecot: managesieve-login: Aborted login (auth failed, 1 attempts in 2 secs): user=<firstname.lastname@example.org>, method=PLAIN, rip=192.168.224.42, lip=192.168.224.42, TLS, session=<P3sZ5aaSCsfCXuAI>
Make sure the user noted in the above log entry matches the user in
/etc/dovecot/master-users. Even the domain part has to match. After editing this file, restart dovecot by:
systemctl restart dovecot