Apex domains cannot have CNAME records.
Question and response
Question
dig mail.outlook.com. NS
Answer
mail.outlook.com. 3 NS ns2-80.azure-dns.net.
mail.outlook.com. 3 NS ns4-80.azure-dns.info.
mail.outlook.com. 3 NS ns1-80.azure-dns.com.
mail.outlook.com. 3 NS ns3-80.azure-dns.org.
Authority
The response has no authority records.
Additional
ns1-80.azure-dns.com. 2 A 150.171.10.80
ns2-80.azure-dns.net. 2 A 150.171.16.80
ns3-80.azure-dns.org. 2 A 13.107.222.80
ns4-80.azure-dns.info. 2 A 13.107.206.80
ns1-80.azure-dns.com. 2 AAAA 2603:1061:0:70:0:0:0:50
ns2-80.azure-dns.net. 2 AAAA 2620:1ec:8ec:70:0:0:0:50
ns3-80.azure-dns.org. 2 AAAA 2a01:111:4000:70:0:0:0:50
ns4-80.azure-dns.info. 2 AAAA 2620:1ec:bda:70:0:0:0:50
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0