mx.clean-mailbox.com has no CNAME records.
Question and response
Question
dig mx.clean-mailbox.com. NS
Answer
The response has no answer records.
Authority
clean-mailbox.com. 1800 SOA hank.ns.cloudflare.com. dns.cloudflare.com. 2396676937 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0