mail.vtxb.com has no NS records.
NS records for this zone are defined at vtxb.com.
NS records for this zone are defined at vtxb.com.
Question and response
Question
dig mail.vtxb.com. NS
Answer
The response has no answer records.
Authority
vtxb.com. 900 SOA nsa.vtx1.net. fieldopadmin.vtx1.net. 38 10800 3600 2419200 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0