I’ve just completed a clean install of Big Sur. In its previous incarnation, I had several mailboxes which were “on my Mac”. They don’t appear in the new Mail, of course. I’ve found them on a Time Machine backup, in ~/Library/Mail/v8/Mac-jbr, and copied them onto the internal SSD. However, I can’t persuade Mail to import them: it says that no valid mailbox files were found. There are folders which look like mailboxes: 4D.mbox, for example, which contains info.plist and a folder with a name that looks like a UUID, in turn containing Data, which has various numbered subfolders.
Any ideas gratefully received, as ever.