Apex domains cannot have CNAME records.
Question and response
Question
dig g1.rmailgateway.com. NS
Answer
g1.rmailgateway.com. 5 NS ns-1882.awsdns-43.co.uk.
g1.rmailgateway.com. 5 NS ns-644.awsdns-16.net.
g1.rmailgateway.com. 5 NS ns-236.awsdns-29.com.
g1.rmailgateway.com. 5 NS ns-1047.awsdns-02.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0