mxext3.mailbox.org has no CNAME records.
Question and response
Question
dig mxext3.mailbox.org. NS
Answer
The response has no answer records.
Authority
mailbox.org. 782 SOA ns.jpberlin.de. root.jpberlin.de. 2014137785 40000 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0