I am using the Pop 3 Connector with Windows Small Business 2008. Everytime the Pop 3 connecter fetches email it logs the error code 212
Because of transient or unknown failures, one or more messages (2) could not be delivered to on the SMTP server ''localhost''. The messages will be left on the POP3 server. If these failures continue to occur, you can either connect to the POP3 account and retrieve or delete the messages manually, or resolve the problem causing the errors. For more information, see the log file at "C:\Program Files\Windows Small Business Server\Logs\pop3connector\pop3service.log".
That user still recieves email, but I don't know what is being left and how to sort the problem out.