Often Times I find myself wishing there was an Outbox. Sometimes when I reply to a pm, and I don't get a reply back for a while... I start to second guess whether I actually sent the message or not.
Or sometimes I click "send" and think I may have made an important typo (Bin - 4000 when I meant 400)
I think this would be a very simple addition and The 50 Message limit could still be in place.
I think this is a very self explanatory suggestion, so I don't think there is much more for me to say about it.
If you have any questions, by all means, ask away!