mx.comtenidos.com has no CNAME records.
Question and response
Question
dig mx.comtenidos.com. NS
Answer
The response has no answer records.
Authority
comtenidos.com. 872 SOA dns.comtenidos.com. dominios.orange.es. 2016041106 7200 7200 2592000 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0