mail.emailsrvr.com has no CNAME records.
Question and response
Question
dig mail.emailsrvr.com. NS
Answer
The response has no answer records.
Authority
emailsrvr.com. 600 SOA pdns219.ultradns.com. postmaster.emailsrvr.com. 2016053250 3600 600 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0