Apex domains cannot have CNAME records.
Question and response
Question
dig cmail1.com. NS
Answer
cmail1.com. 5 NS ns0.dnsmadeeasy.com.
cmail1.com. 5 NS ns3.dnsmadeeasy.com.
cmail1.com. 5 NS ns4.dnsmadeeasy.com.
cmail1.com. 5 NS ns1.dnsmadeeasy.com.
cmail1.com. 5 NS ns2.dnsmadeeasy.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0