clients.mysmtp.com has no CNAME records.
Question and response
Question
dig clients.mysmtp.com. NS
Answer
The response has no answer records.
Authority
mysmtp.com. 3234 SOA pdns01.domaincontrol.com. dns.jomax.net. 2026011907 28800 7200 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0