vopmail.netos.com has no CNAME records.
Question and response
Question
dig vopmail.netos.com. NS
Answer
The response has no answer records.
Authority
netos.com. 10662 SOA ns-1.nuoz.net. postmaster.nwnexus.com. 2024052901 7200 3600 864000 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0