mail.vtxb.com has no CNAME records.
Question and response
Question
dig mail.vtxb.com. NS
Answer
The response has no answer records.
Authority
vtxb.com. 900 SOA nsb.vtx1.net. fieldopadmin.vtx1.net. 38 10800 3600 2419200 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0