Every once in a while the POP3 Connector service stops responding.
To date, we have not found any indication of what failed in the service in the logs, even when the logging level was set to Maximum.
When troubleshooting the Connector, make sure to have the ISP credentials for one of your users so you can monitor their inbox.
Restart the service to see if this fixes the problem. Generally it doesn't.
Consistently, we have to reboot the box.
As a rule, we see if we can have an MX record with the highest priority for the client's domain pointing to the SBS static IP to mitigate this problem as well as eliminate the 15 minute lag the client experiences when all e-mail comes in via the ISP.
To complete the MX process, if the ISP is willing, one will need to have the reverse DNS record set for the IP of the SBS box too. This eliminates the possibility that the e-mail will get stopped by spam filters that do a reverse DNS lookup against the sending SMTP server.
Microsoft Small Business Specialists