mx.rostelecom-cc.RU has no NS records.
NS records for this zone are defined at rostelecom-cc.RU.
NS records for this zone are defined at rostelecom-cc.RU.
Question and response
Question
dig mx.rostelecom-cc.RU. NS
Answer
The response has no answer records.
Authority
rostelecom-cc.ru. 3600 SOA ns.rostelecom-cc.ru. dns.rostelecom-cc.ru. 2829247455 28800 7200 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0