(This probably includes the Outbox as well - though not drafts so if you save a message part way through creation you can get back to it.)
I would draw the conclusion that the Staff use the Private message system to keep in touch but without explicit instructions - which I feel are likely to follow in the next message which will then lock this thread - us mortals are probably not intended to use it.
Jomin
PS. Mind you, this almost un-usability might be related to a general attempt to make things more secure and robust so the most important thing - the Game itself - can be keep going to the max.
PPS. I briefly wondered if, not being able to read messages sent to ourselves, we would receive messages sent by those who CAN send them - but somehow I think a way will be found - even if the body of such a missive is engraved in small print on a black (thunder?) bolt sent from a great height at a high velocity...