tmxmailsmtp.telmex.com has no NS records.
NS records for this zone are defined at telmex.com.
NS records for this zone are defined at telmex.com.
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. 657241150 10800 3600 604800 300
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0