Apex domains cannot have CNAME records.
Question and response
Question
dig mail.monaco.mc. NS
Answer
mail.monaco.mc. 4 NS ns2.monaco-telecom.net.
mail.monaco.mc. 4 NS ns1.monaco-telecom.mc.
Authority
The response has no authority records.
Additional
ns2.monaco-telecom.net. 4 A 195.78.6.210
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0