Apex domains cannot have CNAME records.
Question and response
Question
dig pc5.atmailcloud.com. NS
Answer
pc5.atmailcloud.com. 5 NS ns-1680.awsdns-18.co.uk.
pc5.atmailcloud.com. 5 NS ns-644.awsdns-16.net.
pc5.atmailcloud.com. 5 NS ns-36.awsdns-04.com.
pc5.atmailcloud.com. 5 NS ns-1047.awsdns-02.org.
Authority
The response has no authority records.
Additional
ns-36.awsdns-04.com. 1 A 205.251.192.36
ns-644.awsdns-16.net. 1 A 205.251.194.132
ns-1047.awsdns-02.org. 1 A 205.251.196.23
NS-1680.AWSDNS-18.CO.UK. 1 A 205.251.198.144
ns-36.awsdns-04.com. 1 AAAA 2600:9000:5300:2400:0:0:0:1
ns-644.awsdns-16.net. 1 AAAA 2600:9000:5302:8400:0:0:0:1
ns-1047.awsdns-02.org. 1 AAAA 2600:9000:5304:1700:0:0:0:1
NS-1680.AWSDNS-18.CO.UK. 1 AAAA 2600:9000:5306:9000:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0