Apple Mail: folders deletion

Are these folders stored on your Mac, or is this an IMAP or Exchange account? If it is an IMAP or Exchange account, folders may be stored on the server. If any are stored on your Mac, they are on Users -> your username -> Library -> Mail and Mail downloads, unless you are using something other than Apple's Mail program.

You can basically remove all accounts in Mail menu -> Accounts, but do so only when you have stored your local mail folders outside of your Library folder when Mail is not open. Note, Mail does not always close unless you select Quit from the Mail menu.

If they are folders in the On My Mac section, you can select them all, ctrl-click on the selection, then Delete Mailbox.

To select them all, select the first, scroll to the end, then Shift-Click on the last one.

And yes, it might take a while to delete them all, but you don't have to do them individually.

If you delete them in the Finder, you should also delete the Envelope Index in MailData folder. Then open Mail and it will rebuild everything. If you don't delete the index, it will generated a bunch of recovered mailboxes.

Delete all of them, but not necessary if you do it from within Mail. Deleting the Folders in Finder and then rebuilding the index will take a lot of time, too.