mail.rdtc.RU has no NS records.
NS records for this zone are defined at rdtc.RU.
NS records for this zone are defined at rdtc.RU.
Question and response
Question
dig mail.rdtc.RU. NS
Answer
The response has no answer records.
Authority
rdtc.ru. 3600 SOA ns.rdtc.ru. root.rdtc.ru. 3008041043 5400 1800 604800 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0