Apex domains cannot have CNAME records.
Question and response
Question
dig customeriomail.com. NS
Answer
customeriomail.com. 5 NS ns-577.awsdns-08.net.
customeriomail.com. 5 NS ns-152.awsdns-19.com.
customeriomail.com. 5 NS ns-1865.awsdns-41.co.uk.
customeriomail.com. 5 NS ns-1432.awsdns-51.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0