*Warning: Pre-queue content-filter connection overload
At first I was concerned that the pre-queue content-filtering subsystem of postfix was somehow being overwhelmed and I was possibly loosing mail. Digging around a bit more lead to these types of log entries that seemed like they were the subject of the logwatch report:
Feb 21 13:01:45 pyxidis postfix/smtpd: connect from unknown[unknown]
Feb 21 13:01:45 pyxidis postfix/smtpd: lost connection after CONNECT from unknown[unknown]
Feb 21 13:01:45 pyxidis postfix/smtpd: disconnect from unknown[unknown]
From what I can glean here the log entries above indicate that a SMTP connection was established with the kernel but the connecting host hot potatoed it before postfix was able to process the connection.
When postfix tries to process the connection there's nobody home because the kernel had already removed the connection and it dumps something like the lines above to the mail log.
Here's the scoop from the logwatch docs:
This sometimes occurs in reaction to a portscan or broken bots, or when postfix is overloaded, due to excessive header_checks / body_checks content filtering, or even too few smtpd processes to service the demand. One could reduce the number of header_checks and body_checks, and possibly set smtpd_timeout to 60 (seconds). The key is that existing clients are overloading the number of smtpd daemons. The postfix-logwatch section configuration variable is postfix_ConnectionLostOverload, and the command line option is --connectionlostoverload. If you consider this sub-section to be meaningless, set the level limiter value to 0 and the sub-section will be suppressed.
I was not going to change my header or body checks (because they keep the unwashed spammers at bay) so I opted for tuning my smtpd_timeout down to 30 seconds in main.cf.
Because this looks like it is a possible resource issue you could also up the amount of smtpd processes allowed to service the pre-queue content-filtering subsystem.