"421 4.4.1 Connection timed out" from Exchange 2007

Exchange closed the SMTP connection before POPcon finished transmitting the message. This usually happens with very large messages or a slow network.

Fix

Increase the connection timeout on the Exchange receive connector using the Exchange Management Shell:

Set-ReceiveConnector -id "*default*" -ConnectionTimeout 01:00:00

Then restart the Microsoft Exchange Transport service for the change to take effect.