dalle FAQ di virtualmail-pop3d:

What does "Socket closed" log message mean?

Basically, this means that the POP3 session was not properly closed.
Probably the POP3 client did not send a QUIT command. This is common
with Outlook and Outlook Express. A similar message from another POP3
server is: "-ERR POP EOF or I/O Error".
Il pratica il client ha chiuso la connessione senza inviare il comando QUIT al server POP3.


http://www.reedmedia.net/software/virtualmail-pop3d/FAQ