Apex domains cannot have CNAME records.
Question and response
Question
dig ops.atmailcloud.com. NS
Answer
ops.atmailcloud.com. 5 NS ns-847.awsdns-41.net.
ops.atmailcloud.com. 5 NS ns-1087.awsdns-07.org.
ops.atmailcloud.com. 5 NS ns-1735.awsdns-24.co.uk.
ops.atmailcloud.com. 5 NS ns-356.awsdns-44.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0