Auth failed mailserver

Nextcloud version (eg, 20.0.5): 25.0.3
Operating system and version (eg, Ubuntu 20.04): Ubuntu 20.04
Apache or nginx version (eg, Apache 2.4.25): Nginx 1.18.0-0ubuntu1.4
PHP version (eg, 7.4): PHP 7.4.33

Hello, I hope I am in the right place to raise my problem.
I have many entries on my local mail server originating from nextcloud:

/var/log/syslog.1:Feb 15 23:38:28 mail dovecot: imap-login: Disconnected (auth failed, 2 attempts in 12 secs): user=, method=PLAIN,
/var/log/syslog.1:Feb 15 23:38:30 mail dovecot: imap-login: Disconnected (auth failed, 4 attempts in 20 secs): user=, method=PLAIN,
/var/log/syslog.1:Feb 15 23:38:42 mail dovecot: imap-login: Disconnected (auth failed, 4 attempts in 8 secs): user=, method=PLAIN,

Hello, I hope I am in the right place to raise my problem.

I have many entries on my local mail server originating from nextcloud: