authentication failed: (reason unavailable)

krll-kov

Hi, sometimes I can not receive emails from some websites like gate.io, nord vpn. And regular emails from more common mail servers work good

In log in always see this output

Message
disconnect from unknown[81.30.107.136] ehlo=1 auth=0/1 rset=1 commands=⅔
06/27/2025, 11:30:57 AM info
Message
lost connection after AUTH from unknown[81.30.107.136]
06/27/2025, 11:30:57 AM info
Message
disconnect from unknown[81.30.107.33] ehlo=1 auth=0/1 rset=1 commands=⅔
06/27/2025, 11:30:57 AM info
Message
lost connection after AUTH from unknown[81.30.107.33]
06/27/2025, 11:30:57 AM warning
Message
warning: unknown[81.30.107.33]: SASL LOGIN authentication failed: (reason unavailable), sasl_username=pca
06/27/2025, 11:30:57 AM warning
Message
warning: unknown[81.30.107.136]: SASL LOGIN authentication failed: (reason unavailable), sasl_username=si
06/27/2025, 11:30:49 AM info
Message
Anonymous TLS connection established from unknown[81.30.107.136]: TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)
06/27/2025, 11:30:48 AM info
Message
connect from unknown[81.30.107.136]

esackbauer

Those two things are not correlated. What you see in the log is a typical brute force attempt.
You can see that the IP address trying to log on is a “hacker”:
https://www.abuseipdb.com/check/81.30.107.136
Nothing to worry about, the IP will be on the fail2ban list after a few tries.

Regarding mails that you do not get, you have to properly filter your postfix log, and check for bounce mails on the sender side.

Welcome to mailcow and the wonderful complicated world of SMTP!

krll-kov

esackbauer

Oh, thanks!!