CONTINUOUSWAVE

If you send another user a PRIVATE MESSAGE, the message remains in your OUTBOX, until the recipient actually receives the message. Once the message has been received, it will move to your SENT MESSAGE folder. Until the message has been received, and while it is still in your OUTBOX, you can edit the message or even delete it.

Apparently many users ignore private messages, as I have sent more than a dozen that are still in my OUTBOX awaiting the recipient to read them.