mx.INFRAMX.RU has no NS records.
NS records for this zone are defined at INFRAMX.RU.
NS records for this zone are defined at INFRAMX.RU.
Question and response
Question
dig mx.INFRAMX.RU. NS
Answer
The response has no answer records.
Authority
INFRAMX.RU. 300 SOA ns5.o7.com. admin.o7.com. 24 600 10800 2419200 600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0