IMP is a PHP/MySQL Webmail solution and is quite common, at least on university systems. I recently found out you could use it to download mailbox files in standard Unix mbox format. OS X mistakes these files for actual Mail.app mailboxes (giving them mailbox icons), but you can't directly put them in ~/Library/Mailboxes/. I was still able to transfer the folder using the import procedure, but all messages are marked as new. So this might be more of a proof-of-concept thing than the best way to transfer mail. In fact, it might be easier to access the folder through Mail.app and move all messages to a convenient local mailbox ("On My Mac"). In any event, here's the procedure. In IMP 3.2.1, do:
- Click "Folders"
- Select folder to export
- In the "Choose Action" menu, choose "Download Folder(s)"
- Select "Import Mailboxes" in the "File" menu
- Choose "Standard mbox files"
- Press the '»' button twice to get to standard file dialog
- Select the folder containing your downloaded *.mbox file and press "Choose"
- In the newly-created "Import" or "Import-<N>" folder, you find your imported mbox which you can rename, move, etc.
Let's hope it helps someone.

