If you receive an error message like:
Subject: VPOP3 Alert - Non-existent user - <username> Delivery Failure Report. There was incoming mail for non-existent user "<username>".
This means that VPOP3 downloaded a message using POP3 and it was addressed to an email address which does not exist.
For instance, if a downloaded message had a header containing:
To: bob@yourdomain.com
then VPOP3 will look for a Mapping, User or List called 'bob'. If it can't find one, then it will generate this error message.