First check contents of your files in /mail/new to make sure there is no important mail.

On my cPanel account i have no email account set so when my website send an email to mailbox that dont exist or any other error, the mail is returned there into /mail/new filling space of my hosting account.

So i decided to automatically delete all this mail instead of saving it in /mail/new .

How to automatically delete unrouted emails in cPanel?

There is how to set auto deleting of these emails from within cPanel:

cPanel / Default Address / Advanced Options / select "Discard (Not Recommended)" / Change