- During SMTP session server received unknown or unsupported command. In
/var/log/maillogerrors can be found:
CONFIG_TEXT: postfix/smtpd: too many errors after UNKNOWN from xxxxxxxx.example.com[203.0.113.2]
- No other issues with mail transmission functionality has been detected
The remote host sends a lot of unknown commands.
Inspect the suspicious behavior on the remote host mentioned in the error messages if it is possible.
As a workaround, IP address of the remote host could be added to the server black list (for example via Plesk Firewall).