Apex domains cannot have CNAME records.
Question and response
Question
dig atmailsvr.net. NS
Answer
atmailsvr.net. 5 NS dns3.p06.nsone.net.
atmailsvr.net. 5 NS dns2.p06.nsone.net.
atmailsvr.net. 5 NS dns1.p06.nsone.net.
atmailsvr.net. 5 NS dns4.p06.nsone.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0