mail.rossilber.com has no NS records.
NS records for this zone are defined at rossilber.com.
NS records for this zone are defined at rossilber.com.
Question and response
Question
dig mail.rossilber.com. NS
Answer
The response has no answer records.
Authority
rossilber.com. 600 SOA ns3-l2.nic.ru. dns.nic.ru. 2016112624 1440 3600 2592000 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0