Apex domains cannot have CNAME records.
Question and response
Question
dig au-east.atmailcloud.com. NS
Answer
au-east.atmailcloud.com. 5 NS ns-1571.awsdns-04.co.uk.
au-east.atmailcloud.com. 5 NS ns-291.awsdns-36.com.
au-east.atmailcloud.com. 5 NS ns-897.awsdns-48.net.
au-east.atmailcloud.com. 5 NS ns-1446.awsdns-52.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0