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What does a "421 4.4.1 Connection timed out" Exchange 2007 SMTP error message mean?

This error message means that Exchange closed the SMTP connection while POPcon was still transmitting Emails.

Normally this is not severe because POPcon will just continue to transfer the rest of the emails in the next mail cycle.

The cause of this problem is the default ConnectionTimeout parameter of Exchange 2007 receive connectors. By default this timeout is set to 10 Minutes maximum length of any given SMTP session.

You can use this Exchange management shell command to set a longer SMTP connection timeout:

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