mta.serverlet.com has no CNAME records.
Question and response
Question
dig mta.serverlet.com. NS
Answer
The response has no answer records.
Authority
serverlet.com. 3187 SOA ns1.serverlet.com. info.serverlet.com. 2026022291 10000 7200 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0