Apex domains cannot have CNAME records.
Question and response
Question
dig atmailcloud.com. NS
Answer
atmailcloud.com. 5 NS ns-1186.awsdns-20.org.
atmailcloud.com. 5 NS ns-977.awsdns-58.net.
atmailcloud.com. 5 NS ns-49.awsdns-06.com.
atmailcloud.com. 5 NS ns-1879.awsdns-42.co.uk.
Authority
The response has no authority records.
Additional
ns-977.awsdns-58.net. 2 A 205.251.195.209
ns-1186.awsdns-20.org. 2 A 205.251.196.162
ns-1879.awsdns-42.co.uk. 2 A 205.251.199.87
ns-49.awsdns-06.com. 2 AAAA 2600:9000:5300:3100:0:0:0:1
ns-977.awsdns-58.net. 2 AAAA 2600:9000:5303:d100:0:0:0:1
ns-1186.awsdns-20.org. 2 AAAA 2600:9000:5304:a200:0:0:0:1
ns-1879.awsdns-42.co.uk. 2 AAAA 2600:9000:5307:5700:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0