mx.comtenidos.com has no NS records.
NS records for this zone are defined at comtenidos.com.
NS records for this zone are defined at comtenidos.com.
Question and response
Question
dig mx.comtenidos.com. NS
Answer
The response has no answer records.
Authority
comtenidos.com. 900 SOA dns.comtenidos.com. dominios.orange.es. 2016041106 7200 7200 2592000 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0