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