Apex domains cannot have CNAME records.
Question and response
Question
dig mxrecord.mx. NS
Answer
mxrecord.mx. 5 NS poppy.ns.cloudflare.com.
mxrecord.mx. 5 NS vicky.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0