mx.noris.net has no NS records.
NS records for this zone are defined at noris.net.
NS records for this zone are defined at noris.net.
Question and response
Question
dig mx.noris.net. NS
Answer
The response has no answer records.
Authority
noris.net. 600 SOA dns0.noris.net. hostmaster.noris.net.noris.net. 2012072608 86400 86400 2419200 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0