Using wrong Apple Mail folders

Do either V2 or V3 contain all of the data? I've seen remnants left behind, but they only contained a few files.

I don't see how Mail would be working if it doesn't have the correct folder.

Here is what I would try, but have no way to test this out to see if it would work.

If one of them is holding all the Mail data, then you may be able to rename it. The only plist that I could find with references to the directory names was the PersistenceInfo.plist in the ~/Library/Mail folder. Edit that plist with a text editor and change references to whichever folder is active to V10. There should be two, LastUsedVersionDirectoryName and a key in VersionDirectoryIdentifiers.

Better to use cloning software such as Bombich Carbon Copy Cloner or Shirtpocket Superduper:

Backup now, or forever hold your peace - … - Apple Community

With a clone you can at least reinstall the operating system on the clone and it will retain the same character as the original with all the same files.

Keep in mind, mail folders made under IMAP or Exchange are not stored on your Mac, unless explictly stated to be On My Mac. They generally are only stored at the email host. Only POP is exclusively on your Mac, and is not shared with other phones, tablets, and computers.

You should have a V10 folder. I don’t know how Mail would be working if you don’t. The Mail version folders have updated every major macOS version, so V2 and V3 are very old.