_spf.sdu.dk has no NS records.
NS records for this zone are defined at sdu.dk.
NS records for this zone are defined at sdu.dk.
Question and response
Question
dig _spf.sdu.dk. NS
Answer
The response has no answer records.
Authority
sdu.dk. 900 SOA ns1.sdu.dk. dns.sdu.dk. 2012272628 86400 7200 2419200 900
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0