mx.zsttk.RU has no NS records.
NS records for this zone are defined at zsttk.RU.
NS records for this zone are defined at zsttk.RU.
Question and response
Question
dig mx.zsttk.RU. NS
Answer
The response has no answer records.
Authority
zsttk.ru. 120 SOA ns.zsttk.ru. hostmaster.zsttk.ru. 2020061617 60 30 604800 120
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0