Testing in Thunderbird, the mbox files exported from the delivery module show up with the text of the message blank, but the attachments are there.
The box files include:
Content-Type: text/plain; charset="UTF-8"
It looks like the "boundary" line should be:
i.e. 6 hyphens instead of 4 - which is what's already there for the attachments. Apparently there are supposed to be two leading hyphens plus whatever is defined as the boundary. With that change, I was able to read both single messages and folders in Thunderbird.