Apex domains cannot have CNAME records.
Question and response
Question
dig inbound.protection.outlook.com. NS
Answer
inbound.protection.outlook.com. 5 NS ns3-33.azure-dns.org.
inbound.protection.outlook.com. 5 NS ns4-33.azure-dns.info.
inbound.protection.outlook.com. 5 NS ns1-33.azure-dns.com.
inbound.protection.outlook.com. 5 NS ns2-33.azure-dns.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0