mx21.email-cluster.com has no CNAME records.
Question and response
Question
dig mx21.email-cluster.com. NS
Answer
The response has no answer records.
Authority
email-cluster.com. 1595 SOA elly.ns.cloudflare.com. dns.cloudflare.com. 2394352943 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0