smtpus.telefonica.com has no CNAME records.
Question and response
Question
dig smtpus.telefonica.com. NS
Answer
The response has no answer records.
Authority
telefonica.com. 300 SOA dns2.movistar.es. please_set_email.absolutely.nowhere. 2419 10800 3600 2419200 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0