mx3.mailhostbox.com has no CNAME records.
Question and response
Question
dig mx3.mailhostbox.com. NS
Answer
The response has no answer records.
Authority
mailhostbox.com. 1482 SOA andy.ns.cloudflare.com. dns.cloudflare.com. 2396821469 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0