Apex domains cannot have CNAME records.
Question and response
Question
dig mtpserver.net. NS
Answer
mtpserver.net. 5 NS ns-1456.awsdns-54.org.
mtpserver.net. 5 NS ns-553.awsdns-05.net.
mtpserver.net. 5 NS ns-405.awsdns-50.com.
mtpserver.net. 5 NS ns-1861.awsdns-40.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0