_spf.zygo.nl has no NS records.
NS records for this zone are defined at zygo.nl.
NS records for this zone are defined at zygo.nl.
Question and response
Question
dig _spf.zygo.nl. NS
Answer
The response has no answer records.
Authority
zygo.nl. 86400 SOA auth01.dns.trueserver.nl. hostmaster.trueserver.nl. 2018102495 86400 3600 604800 86400
Additional
. 0 OPT ; payload 1400, xrcode 0, version 0, flags 0