cubedns.com has no MX records.
Question and response
Question
dig cubedns.com. MX
Answer
The response has no answer records.
Authority
cubedns.com. 300 SOA ns1.vultr.com. dnsadm.choopa.com. 0 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0