To be honest I didn't even know we had an outbox for private messages. So I looked it up and found the answer at
https://www.phpbb.com/community/viewtop ... 4&t=675335 The outbox is for messages that haven't been viewed by the recipient yet, and the sender can edit or cancel messages that are still in the outbox. That's a pretty useful feature for someone who sent a message they regret lol.