Stopping "Non-existent user" messages when downloading using POP3

> Please can you tell me how to stop VPOP3 sending "VPOP3 Alert - Non-existent user" email messages to postmaster

This error is sent because there's a routing problem in the POP3 Mail Collector - it can't find who to send the message to. So, you change the error handling behaviour of the Mail Collector in the Mail Collector settings. In the VPOP3 settings, go to Mail Connectors -> Mail Collectors -> (your collector) -> Routing Errors, and choose what you want VPOP3 to do if it can't route a message automatically.