mx3.mailbox.org has no MX records.
Question and response
Question
dig mx3.mailbox.org. MX
Answer
The response has no answer records.
Authority
mailbox.org. 900 SOA ns.jpberlin.de. root.jpberlin.de. 2014137785 40000 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0