[Discuss] Migrating hundreds of mbox files to an IMAP server?

[Daniel Barrett]

What's the best way to import 600 mbox files to an IMAP server?

Background: The files were created locally by Thunderbird, arranged in
a file hierarchy. I'd like the same hierarchy to be created on the
IMAP server. For example, the mbox file named Recipes.mbox would
migrate to a new IMAP folder named Recipes, and the mbox file at path
Foo/Bar/Blat.mbox would migrate to a new IMAP folder "Blat" within
folder "Foo/Bar" on the IMAP server.

I don't have direct filesystem access to the IMAP server. (It's
fastmail.com.)

Things I've tried:

- Fastmail has a web UI for migrating mbox files, which works pretty
well, but it requires manual mousing & typing for every mbox. Doing
this 600 times would be...not fun. :-)

- Dragging the local folders in Thunderbird to the IMAP server doesn't
work.  Thunderbird crashes or just stops copying. (Thunderbird's
instability is a catalyst for this migration.)

- I read about the tool imapsync, but if I understand correctly, it
copies from one IMAP server to another, not from a directory hierarchy
of mbox files to an IMAP server. Perhaps I could spin up a temporary
IMAP service to serve the files just for this migration, but I've
never done this before.

Thank you very much for any ideas!

Dan