smtp4.jazztel.com has no CNAME records.
Question and response
Question
dig smtp4.jazztel.com. NS
Answer
The response has no answer records.
Authority
jazztel.com. 10800 SOA dns.jazztel.es. admin.jazztel.es. 2014105178 86400 7200 2592000 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0