mx1.mailbox.org has no TXT records.
Question and response
Question
dig mx1.mailbox.org. TXT
Answer
The response has no answer records.
Authority
mailbox.org. 900 SOA ns.jpberlin.de. root.jpberlin.de. 2014137874 40000 7200 604800 86400
Additional
. 0 OPT ; payload 1480, xrcode 0, version 0, flags 0