mail.o7.com has no NS records.
NS records for this zone are defined at o7.com.
NS records for this zone are defined at o7.com.
Question and response
Question
dig mail.o7.com. NS
Answer
The response has no answer records.
Authority
o7.com. 300 SOA ns5.o7.com. support.cloud.rt.ru. 2016051535 600 10800 2419200 600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0