smtp01.mbxg.com has no CNAME records.
Question and response
Question
dig smtp01.mbxg.com. NS
Answer
The response has no answer records.
Authority
mbxg.com. 740 SOA dns01.donovandata.com. domainadmin.mediaocean.com. 2024080703 900 600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0