I've successfully been using El Capitan and Mail for a few weeks. Today all but one of my many "On my Mac" Mail folders went missing from the sidebar. The folders are still visible and file sizes indicate content is still there in the library Mail V3 folder. Is there any way to make these folders visible short of reimporting them all? I have several folders with sub folders and sub-sub folders and tons of emails ... trying to avoid the tedious process of reimporting. I've tried creating a new mailbox with the same name to see if the content would automatically appear. It doesn't. I've tried closing the app and restarting the computer. No luck. I do have a TimeMachine backup if that would help in solving this. Any ideas?