Apex domains cannot have CNAME records.
Question and response
Question
dig self-mail-messaging.com. NS
Answer
self-mail-messaging.com. 5 NS ns-1892.awsdns-44.co.uk.
self-mail-messaging.com. 5 NS ns-1479.awsdns-56.org.
self-mail-messaging.com. 5 NS ns-65.awsdns-08.com.
self-mail-messaging.com. 5 NS ns-901.awsdns-48.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0