No.. Not in /var/log/messages, not in /var/log/mysql/mysql-bin.err - /var/log/mysql.log is empty.
It is said that MYSQL logging can be turned on while running..? I don't know how to do it nor found something when googlin'..
Further investigation gave:
/etc/postfix/mysql-virtual_client.cf has access to the Postfix blacklist. The blacklist contained only three rows, but I have deleted them.
I think it could have been a race condition: IPSConfig or Postfix throws two queries at MYSQL at the same time: Maybe to check "sender" with one query and "client" with another almost simultaneously. As there are many postfix processes (anvil, pickup, qmgr etc.) this could likely happen.
And the postfix timeout must be very low.
And mysql_virtual_client.cf is not the only spot the problems occur. Must be similar every time postfix interacts with MySQL. Besides, I have no problem with MySQL. However, 20% of the queries get smashed (!):
max. gleichzeitige Verbindungen 24 --- ---
Fehlversuche 7 12,79 0,58%
Abgebrochen 247 451,14 20,43%
Insgesamt 1,209 2,208,22 100,00%