Apex domains cannot have CNAME records.
Question and response
Question
dig smtp.secureserver.net. NS
Answer
smtp.secureserver.net. 5 NS cns3.secureserver.net.
smtp.secureserver.net. 5 NS cns4.secureserver.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0