macOS Mail Missing Mailbox Folders

Have your Mail folders gone missing on your Mac? Here is something you can try to get them to show up in the left sidebar again.

This quick fix is for the user who has only one email provider set up and it might still be configured to use POP where email gets stored locally in the On My Mac section. (I also have iCloud configured on my Mac, but you can ignore that for the purposes of this demonstration.)