Apex domains cannot have CNAME records.
Question and response
Question
dig stackmail.com. NS
Answer
stackmail.com. 5 NS ns1.stackdns.com.
stackmail.com. 5 NS ns2.stackdns.com.
stackmail.com. 5 NS ns3.stackdns.com.
stackmail.com. 5 NS ns4.stackdns.com.
Authority
The response has no authority records.
Additional
ns1.stackdns.com. 5 A 35.197.225.59
ns2.stackdns.com. 5 A 146.148.28.88
ns3.stackdns.com. 5 A 35.198.79.191
ns4.stackdns.com. 5 A 104.196.137.209
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0