The cPanel main email user account has a speciality. It is actually a server related mail address. Specifically it is an alias to the the mail address "cpanel_user@server_hostname".
Once thing I did notice under cPanel was that when you set the catch-all address to a non-existent one, then the all mails send to the default mail address will fail. You would get a bounce-back stating :
"unrouteable mail address"
FIX : Simply reset the catch-all address as the default address.