mx.jazztel.es has no CNAME records.
Question and response
Question
dig mx.jazztel.es. NS
Answer
The response has no answer records.
Authority
jazztel.es. 7020 SOA dns.jazztel.es. admin.jazztel.es. 2014061466 7200 7200 2419200 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0