-ERR Unknown User is a POP3 error. There are two places you can receive it:
If you receive it as part of a 'POP3 Client Error' email message from VPOP3, then it means that VPOP3 was unable to log on to your ISP's mail server because the ISP POP3 server reported an error “Unknown User”. In this case, check the In Mail settings in VPOP3 as the error means that you have specified the wrong 'User ID' (check with your ISP if you are not sure)
If you receive it as an error message in your email client (usually in a popup window) then it means that the email client is trying to connect to VPOP3 using a user name which VPOP3 does not recognise. Check that you have configured the email client correctly with the username and password of a User in the VPOP3 User tab.