smtp00.messageflow.com has no CNAME records.
Question and response
Question
dig smtp00.messageflow.com. NS
Answer
The response has no answer records.
Authority
messageflow.com. 1555 SOA athena.ns.cloudflare.com. dns.cloudflare.com. 2397753105 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0