tmxmailsmtp.telmex.com has no CNAME records.
Question and response
Question
dig tmxmailsmtp.telmex.com. NS
Answer
The response has no answer records.
Authority
telmex.com. 300 SOA ns8.telmex.com. dominios.telmex.com. 657239976 10800 3600 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0