smtp16a.msgapp.com has no CNAME records.
Question and response
Question
dig smtp16a.msgapp.com. NS
Answer
The response has no answer records.
Authority
msgapp.com. 723 SOA ns-1377.awsdns-44.org. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0