Apex domains cannot have CNAME records.
Question and response
Question
dig pc4.atmailcloud.com. NS
Answer
pc4.atmailcloud.com. 5 NS ns-1224.awsdns-25.org.
pc4.atmailcloud.com. 5 NS ns-306.awsdns-38.com.
pc4.atmailcloud.com. 5 NS ns-1015.awsdns-62.net.
pc4.atmailcloud.com. 5 NS ns-1992.awsdns-57.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0