clusterdns.org has no MX records.
Question and response
Question
dig clusterdns.org. MX
Answer
The response has no answer records.
Authority
clusterdns.org. 1800 SOA aron.ns.cloudflare.com. dns.cloudflare.com. 2396879067 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0