mailrelay4.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 mailrelay4.nrb.be. NS
Answer
The response has no answer records.
Authority
nrb.be. 600 SOA ns6.nrb.be. hostmaster.nrb.be. 2016054003 900 180 259200 900
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0