mx.obtelecom.ru has no NS records.
NS records for this zone are defined at obtelecom.ru.
NS records for this zone are defined at obtelecom.ru.
Question and response
Question
dig mx.obtelecom.ru. NS
Answer
The response has no answer records.
Authority
obtelecom.ru. 3600 SOA ns.obcity.ru. dennsk.gmail.com. 2016070416 3600 900 3600000 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0