Inbox
This is where the messages you receive are kept. At the moment I think you can only store a maximum of 50. Should this be increased? PMs may not be recieved until you delete some.
This can also be set to automatically delete the oldest message for you.
Sent
Messages you have sent to other forum users, which have been received, are stored here. When you hit the 50 message cap for the folder the oldest message is deleted automatically. I don't think I can change this.
Outbox
When you send a message it is firstly stored in the outbox. It's stored here until it is read by the recipient. When the recipient reads the message it is then moved into the Sent folder.
This is clever by design as on some forums 'flame wars' happen and people send messages they sometimes regret.
Deleting a message from the outbox will also delete it from the recipient's inbox.
