Hello.
I have a similar problem: Postfix and Dovecot doesn't recognize users from my database.
May 24 14:17:58 plesk01 dovecot: pop3-login: Disconnected (auth failed, 1 attempts in 2 secs): user=<
[email protected]>, method=PLAIN, rip=xxx.xxx.xxx.xxx, lip=10.240.0.2...