Apex domains cannot have CNAME records.
Question and response
Question
dig rmailgateway.com. NS
Answer
rmailgateway.com. 5 NS ns-1490.awsdns-58.org.
rmailgateway.com. 5 NS ns-359.awsdns-44.com.
rmailgateway.com. 5 NS ns-653.awsdns-17.net.
rmailgateway.com. 5 NS ns-1949.awsdns-51.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0