dns01.cloudoon.com has no MX records.
Question and response
Question
dig dns01.cloudoon.com. MX
Answer
The response has no answer records.
Authority
cloudoon.com. 1800 SOA hugh.ns.cloudflare.com. dns.cloudflare.com. 2397162411 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0