imap.vtxb.com has no CNAME records.
Question and response
Question
dig imap.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 1232, xrcode 0, version 0, flags 0