Apex domains cannot have CNAME records.
Question and response
Question
dig zamailgate.com. NS
Answer
zamailgate.com. 4 NS ns-1888.awsdns-44.co.uk.
zamailgate.com. 4 NS ns-86.awsdns-10.com.
zamailgate.com. 4 NS ns-669.awsdns-19.net.
zamailgate.com. 4 NS ns-1524.awsdns-62.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0