mail-ru-sender01b.prod.mail.amosrv.ru has no NS records.
NS records for this zone are defined at mail.amosrv.ru.
NS records for this zone are defined at mail.amosrv.ru.
Question and response
Question
dig mail-ru-sender01b.prod.mail.amosrv.ru. NS
Answer
The response has no answer records.
Authority
mail.amosrv.ru. 600 SOA ns1.amosrv.ru. admin.amosrv.ru. 1774016771 604800 86400 2419200 604800
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0