Administrator receives a bounce email message from Plesk server with the below text:
This address no longer accepts mail.
Email address from the bounce message does not exist in Plesk at Domains > example.com > Email Addresses.
Action What to do with mail for non-existent users is set to Bounce with message at Domains > example.com > Mail Settings page:
Emails sent to non-existing email addresses are set to be bounced.
This is the expected behavior for such settings. The bounce messages can be safely ignored.