mx.dominios.telefonica.net has no CNAME records.
Question and response
Question
dig mx.dominios.telefonica.net. NS
Answer
The response has no answer records.
Authority
telefonica.net. 607 SOA dns2.movistar.es. admondns.tsm.es. 2017050588 10800 3600 2419200 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0