When having especially SBS 2008 with the POP3 Connector to retrieve mail from an outside mailbox to your internal Exchange server, you may encounter the below error in the Event Viewer.

An error occurred while delivering mail to ‘mailbox@mydomain.com’ on the SMTP server ‘localhost’. The error code was 0x800ccc6c. Verify that the mailbox exists and that the Exchange receive connectors are configured properly.

Thou the error message is misleading, this problem can happen if the POP3 Protocol Errors meet the tolerance of the Exchange SMTP Receive connectors. Exchange will close an SMTP connection after a certain number of protocol errors. The default is 5.

To solve the issue, in my case I had to in increase the value of “MaxProtocolErrors” for the Exchange SMTP Receive connectors from Exchange Management Shell by using the below command.

Set-ReceiveConnector -identify “Name of Connector” -MaxProtocolErrors 999

After this, I have restarted the POP3 Connector retrieval of messages and it worked.