Apex domains cannot have CNAME records.
Question and response
Question
dig coxmail.com. NS
Answer
coxmail.com. 3 NS ns.west.cox.net.
coxmail.com. 3 NS ns.east.cox.net.
coxmail.com. 3 NS ns.cox.net.
Authority
The response has no authority records.
Additional
ns.cox.net. 3 A 98.163.68.36
ns.east.cox.net. 3 A 98.163.68.37
ns.west.cox.net. 3 A 98.163.68.52
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0