What is `lost connection after CONNECT from unknown[unknown]`?

Log entries like these:

Jun 13 20:25:10 v554 postfix/smtpd[22126]: lost connection after CONNECT from unknown[unknown] 
Jun 13 20:25:10 v554 postfix/smtpd[22126]: disconnect from unknown[unknown] 
Jun 13 20:25:33 v554 postfix/smtpd[22126]: connect from unknown[unknown] 
Jun 13 20:25:33 v554 postfix/smtpd[22126]: lost connection after CONNECT from unknown[unknown] 
Jun 13 20:25:33 v554 postfix/smtpd[22126]: disconnect from unknown[unknown]

indicate that the client disconnects before Postfix handles the connection.
The kernel threw away the endpoint information when the client disconnected -- and before Postfix had time to handle the connection.
This could also be a symptom of doing too much before-queue processing: When Postfix falls behind, established connections accumulate in the kernel, and clients disconnect after timeout while waiting for the SMTP server to respond.
In other words, those log entries are very unlikely to be seen for legitimate clients - unless there's something in front of Postfix which somehow handles the connection before Postfix can accept it.

© by Ralf Hildebrandt
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This file was last modified 04. Aug 2007 by root

What is lost connection after CONNECT from unknown[unknown]?

What is `lost connection after CONNECT from unknown[unknown]`?