mail.alestra.com.mx has no NS records.
NS records for this zone are defined at alestra.com.mx.
NS records for this zone are defined at alestra.com.mx.
Question and response
Question
dig mail.alestra.com.mx. NS
Answer
The response has no answer records.
Authority
alestra.com.mx. 3600 SOA dns.alestra.net.mx. soporteip.axtel.com.mx. 762215073 10800 3600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0