Each message can be proccessed one or not this one. Processing of income message means that the user read it. Processing of outcome message means that the user send it. At the each list unprocessed messages are marked. It is useful to show user the count of unprocessed messages. This number should calculate at the moment while user request is processed. So it makes the user will be notified as soon as new message was received.
In particular, unproceed outbox message is marked as draft and can be edited and sent later.