b.mx.broadviewnet.net has no CNAME records.
Question and response
Question
dig b.mx.broadviewnet.net. NS
Answer
The response has no answer records.
Authority
mx.broadviewnet.net. 10579 SOA ns5-auth.windstream.net. hostmaster.windstream.net. 2020100600 28800 7200 3600000 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0