mail.dc3.cz has no NS records.
NS records for this zone are defined at dc3.cz.
NS records for this zone are defined at dc3.cz.
Question and response
Question
dig mail.dc3.cz. NS
Answer
The response has no answer records.
Authority
dc3.cz. 600 SOA ns1.tele3.cz. root.tele3.cz. 1772779236 28800 7200 1209600 43200
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0