mx bounces do not correctly display the original message
if you send a message to a non-existent account in a leap provider, you will get a bounce, sent by leap-mx. leap-mx seems to be correctly attaching the original message, but Thunderbird doesn't seem to be able to display.
debug this, is it a leap-mail or a mx problem?
(from redmine: created on 2016-04-01, relates #7943 (closed))