Apex domains cannot have CNAME records.
Question and response
Question
dig xmailerserver.net. NS
Answer
xmailerserver.net. 4 NS ns-2020.awsdns-60.co.uk.
xmailerserver.net. 4 NS ns-406.awsdns-50.com.
xmailerserver.net. 4 NS ns-993.awsdns-60.net.
xmailerserver.net. 4 NS ns-1174.awsdns-18.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0