If your mail queue has too many undeliverable messages it can delay the delivery of new legitimate email. Emails can be deleted from the Plesk control panel, but with very large queues this can be very time consuming. The following command can be used to search for a given subject and delete the mail that matches.
/usr/local/psa/admin/sbin/mailqueuemng -Sfailure\ notice
This would search for all email with "failure notice" in the subject and delete them. If you want to delete all mail in the mail queue you can run the command: