Apex domains cannot have CNAME records.
Question and response
Question
dig us-east.atmailcloud.com. NS
Answer
us-east.atmailcloud.com. 5 NS ns-1556.awsdns-02.co.uk.
us-east.atmailcloud.com. 5 NS ns-1500.awsdns-59.org.
us-east.atmailcloud.com. 5 NS ns-457.awsdns-57.com.
us-east.atmailcloud.com. 5 NS ns-662.awsdns-18.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0