This is an SMTP error meaning that an attempt to authenticate has failed.

If this error is appearing when the email client is trying to log into VPOP3, then you can check the VPOP3 'SECURITY.LOG' file which may contain more information, such as an invalid password, unknown username, etc. (This information is not returned in the SMTP session because it would help an attacker).

If this error is appearing when VPOP3 is trying to log onto your Internet provider, then check the required authentication details with your Internet provider and check they are correct on the Mail Connectors → Mail Senders page in the VPOP3 settings.