Here is the /var/log/maillog content:
Nov 23 15:17:56 server dovecot[1999]: auth: Error: plain(
[email protected],127.0.0.1,<qSz8GQKYhKt/AAAB>): user not found from any userdbs
Nov 23 15:17:56 server dovecot[1999]: imap: Error: auth-master: login: request [2787639297]: Login auth request failed...