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.