_spf.nrb.be has no NS records.
NS records for this zone are defined at nrb.be.
NS records for this zone are defined at nrb.be.
Question and response
Question
dig _spf.nrb.be. NS
Answer
The response has no answer records.
Authority
nrb.be. 600 SOA ns6.nrb.be. hostmaster.nrb.be. 2016053999 900 180 259200 900
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0