Apex domains cannot have CNAME records.
Question and response
Question
dig mailcore.net. NS
Answer
mailcore.net. 5 NS pns24.cloudns.net.
mailcore.net. 5 NS pns21.cloudns.net.
mailcore.net. 5 NS pns22.cloudns.net.
mailcore.net. 5 NS pns23.cloudns.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0