mx1.rivnet.com has no NS records.
NS records for this zone are defined at rivnet.com.
NS records for this zone are defined at rivnet.com.
Question and response
Question
dig mx1.rivnet.com. NS
Answer
The response has no answer records.
Authority
rivnet.com. 900 SOA nsb.vtx1.net. fieldopadmin.vtx1.net. 160 10800 3600 2419200 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0