Apex domains cannot have CNAME records.
Question and response
Question
dig nomail.mx. NS
Answer
nomail.mx. 5 NS ns-1227.awsdns-25.org.
nomail.mx. 5 NS ns-792.awsdns-35.net.
nomail.mx. 5 NS ns-261.awsdns-32.com.
nomail.mx. 5 NS ns-1725.awsdns-23.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0