mail.vtx1.net has no NS records.
NS records for this zone are defined at vtx1.net.
NS records for this zone are defined at vtx1.net.
Question and response
Question
dig mail.vtx1.net. NS
Answer
The response has no answer records.
Authority
vtx1.net. 900 SOA nsb.vtx1.net. fieldopadmin.vtx1.net. 494 10800 3600 2419200 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0