mail.o7n.nl has no NS records.
NS records for this zone are defined at o7n.nl.
NS records for this zone are defined at o7n.nl.
Question and response
Question
dig mail.o7n.nl. NS
Answer
The response has no answer records.
Authority
o7n.nl. 300 SOA ns0.transip.net. hostmaster.transip.nl. 2026011600 86400 1800 2419200 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0