Apex domains cannot have CNAME records.
Question and response
Question
dig eu-west.atmailcloud.com. NS
Answer
eu-west.atmailcloud.com. 5 NS ns-312.awsdns-39.com.
eu-west.atmailcloud.com. 5 NS ns-1429.awsdns-50.org.
eu-west.atmailcloud.com. 5 NS ns-789.awsdns-34.net.
eu-west.atmailcloud.com. 5 NS ns-1977.awsdns-55.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0