Apex domains cannot have CNAME records.
Question and response
Question
dig eu1.rmailgateway.com. NS
Answer
eu1.rmailgateway.com. 5 NS ns-1399.awsdns-46.org.
eu1.rmailgateway.com. 5 NS ns-153.awsdns-19.com.
eu1.rmailgateway.com. 5 NS ns-937.awsdns-53.net.
eu1.rmailgateway.com. 5 NS ns-1936.awsdns-50.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0