mx.obcity.ru has no NS records.
NS records for this zone are defined at obcity.ru.
NS records for this zone are defined at obcity.ru.
Question and response
Question
dig mx.yandex.ru. NS
Answer
The response has no answer records.
Authority
yandex.ru. 900 SOA ns1.yandex.ru. sysadmin.yandex-team.ru. 2023037885 600 300 2592000 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0