I have additional Inbox, Outbox, Sent etc in my Local Folders. They have nothing in them but I can’t delete them. I suspect they appeared when I imported from a previous installation. It’s not serious but it would be nice to remove them or at least hide them but they won’t hide either.
Unfortunately the system folders (Inbox, Sent etc.) are default for Local Folders, and you cannot delete them.
If you don’t want to see one of them, right-click on it and choose Hide. It will then be banished to the More folder.