mailhost.telefonica.net has no NS records.
NS records for this zone are defined at telefonica.net.
NS records for this zone are defined at telefonica.net.
Question and response
Question
dig mailhost.telefonica.net. NS
Answer
The response has no answer records.
Authority
telefonica.net. 900 SOA dns2.movistar.es. admondns.tsm.es. 2017050588 10800 3600 2419200 900
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0