To delete all undeliverable emails for just one domain on your GoPanel server
$ /go/undelivered gopanel.org

To completely clear all undeliverable emails from your entire GoPanel server (all domains)
$ /go/undelivered all

To filter undeliverable emails and save them to a database instead of simply deleting them
$ /go/undelivered
$ sqlite3 /panel/database/undelivered.db "SELECT Email FROM Undelivered ORDER BY Email"