Symptoms

Backup restore fails:

Execution of C:\\webservices\\plesk\\bin\\mail.exe --create login@example.com -mailbox false -mbox_quota -1 -forwarding true -cp-access true -forwarding-addresses add:forwardhere@example.com -ignore-nonexistent-options failed with return code 1.
    Stderr is
    An error occurred during mail account creation: Unable to create user to mailname. The password should be specified.

If there are mailboxes on the server with CP (Control Panel) access but with no Mailbox, they are not restored.

Cause

This issue has been considered as software issue with ID #PPPM-4855 that was fixed in Plesk Onyx.

Resolution

Upgrade to the latest Plesk version .