mx.nic.cz has no NS records.
NS records for this zone are defined at nic.cz.
NS records for this zone are defined at nic.cz.
Question and response
Question
dig mx.nic.cz. NS
Answer
The response has no answer records.
Authority
nic.cz. 1800 SOA a.ns.nic.cz. hostmaster.nic.cz. 1772183662 14400 3600 1209600 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0