Outbox only exists if you send mail "later" (ctrl+shift+Enter) as opposed to send now (ctrl+Enter) it should usually be empty and resides exclusively in the local folders account, although I did see a bug about moving that to one per email account, but I do not think that has been implemented as yet.

However if something has deleted it, sending an email later will recreate it.

I just did a test of Send Later, using it for the first time ever. I couldn't find where the message went, but a little research showed that it should be under Local Folders then Outbox. I don't have either Local Folders or Outbox. I may have deleted Local Folders/Outbox at some point when "cleaning up" my Thunderbird window. Is there a way to get Local Folders and Outbox back?

Please disregard my previous message. I found Local Folders and Outbox. Sorry for any inconvenience.