Apex domains cannot have CNAME records.
Question and response
Question
dig fortimailcloud.com. NS
Answer
fortimailcloud.com. 5 NS ns4.fortinet.com.
fortimailcloud.com. 5 NS ns-a1.fortinet.com.
fortimailcloud.com. 5 NS ns-o1.fortinet.com.
fortimailcloud.com. 5 NS ns3.fortinet.com.
Authority
The response has no authority records.
Additional
ns3.fortinet.com. 5 A 208.91.113.63
ns4.fortinet.com. 5 A 154.52.16.79
ns-a1.fortinet.com. 5 A 52.41.128.72
ns-o1.fortinet.com. 5 A 38.21.200.118
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0