To save the disk quota, I forward all new incoming emails to an external account. My mail server keep a copy of the message on the server. How can I delete the saved copy using cPanel?
There are 2 ways you can delete the copy of the email that is left on the server. You can do it automatically or manually.
If you want to do it automatically, you should delete the pop3 account and leave the forwarder. A forwarder does not need a pop3 account.
If you want to do it manually, go to the File Manager in cPanel and you will see a directory called Mail in the right-hand column. You can delete the messages there.