mx.optorg.ru has no NS records.
NS records for this zone are defined at optorg.ru.
NS records for this zone are defined at optorg.ru.
Question and response
Question
dig mx.optorg.ru. NS
Answer
The response has no answer records.
Authority
optorg.ru. 3536 SOA ns4.optorg.ru. admin.optorg.ru. 2025101503 3600 900 3600000 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0