Version 6.0 of Postbox came out in April, although I have yet to upgrade. I discovered that were intially using a new default file format:
I also discovered that as of v6.0.9, they reverted to MBOX as their default for IMAP mail. However, I anticipate that while this may be due to some bugginess in compatibility of the format with Thunderbird, that this will eventually be fixed, and they will switch back to Maildir.
So, if this happens, would this format be supported by DT with drag/drop functionality, just as MBOX has? I didn’t see any mention of this format in the manual.