Apex domains cannot have CNAME records.
Question and response
Question
dig pc3.atmailcloud.com. NS
Answer
pc3.atmailcloud.com. 5 NS ns-1775.awsdns-29.co.uk.
pc3.atmailcloud.com. 5 NS ns-1296.awsdns-34.org.
pc3.atmailcloud.com. 5 NS ns-112.awsdns-14.com.
pc3.atmailcloud.com. 5 NS ns-922.awsdns-51.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0