mail.mdmhost.com has no CNAME records.
Question and response
Question
dig mail.mdmhost.com. NS
Answer
The response has no answer records.
Authority
mdmhost.com. 1732 SOA kaiser.ns.cloudflare.com. dns.cloudflare.com. 2396281971 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0