mx.gadsa.es has no CNAME records.
Question and response
Question
dig mx.gadsa.es. NS
Answer
The response has no answer records.
Authority
gadsa.es. 5827 SOA ns1.acens.net. hostmaster.ns1.acens.net. 1771952207 14400 3600 604800 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0