This usually happens if the “VPOP3 AV” virus scanner has a problem it can't automatically recover from. This may be a fatal error, or a licencing issue.
You can check this on the Settings → Antivirus settings page. Look in the “Last 10 items scanned” box to see if any errors are reported there.
If the problem is that the subscription has expired (maybe you were trying the 30 day free evaluation subscription?) then simply turning off the virus scanner should fix the problem.
Otherwise, restarting VPOP3 should resolve the problem. It is worth checking whether there is a later version of the VPOP3 AV plugin available to download from our website.
The default behaviour of VPOP3 is to block new messages arriving in VPOP3 if you have the virus scanner enabled, because otherwise you may receive messages that haven't been scanned for viruses, when you expect them to have been.
If you would prefer VPOP3 to continue accepting new messages even if there is a problem with the virus scanner, you can turn off the
Stop accepting messages after a fatal virus scanner error checkbox.