mail-8.mtpserver.net has no CNAME records.
Question and response
Question
dig mail-8.mtpserver.net. NS
Answer
The response has no answer records.
Authority
mtpserver.net. 900 SOA ns-553.awsdns-05.net. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0