mx2.sipo.nl has no NS records.
NS records for this zone are defined at sipo.nl.
NS records for this zone are defined at sipo.nl.
Question and response
Question
dig mx2.sipo.nl. NS
Answer
The response has no answer records.
Authority
sipo.nl. 86400 SOA ns1.sipo.nl. hostmaster.sipo.nl. 1773916141 14400 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0