Apex domains cannot have CNAME records.
Question and response
Question
dig fortimail.com. NS
Answer
fortimail.com. 4 NS ns4.fortinet.com.
fortimail.com. 4 NS ns-o1.fortinet.com.
fortimail.com. 4 NS ns3.fortinet.com.
Authority
The response has no authority records.
Additional
ns3.fortinet.com. 4 A 208.91.113.63
ns4.fortinet.com. 4 A 154.52.16.79
ns-o1.fortinet.com. 4 A 38.21.200.118
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0