Apex domains cannot have CNAME records.
Question and response
Question
dig customerly-mail.com. NS
Answer
customerly-mail.com. 5 NS ns-358.awsdns-44.com.
customerly-mail.com. 5 NS ns-895.awsdns-47.net.
customerly-mail.com. 5 NS ns-1443.awsdns-52.org.
customerly-mail.com. 5 NS ns-1601.awsdns-08.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0