I think that 50 messages is way too small for private message inbox and outbox combined. At the very least, I think the limit should be 50 each. Sometimes people forget to clean out their messages and can't receive messages because their inbox is full, and on top of it aren't receiving e-mails.
It would be cool if a message could be on standby and notify the receiving member about it once he/she logs onto the forums. But that's probably impossible.
If it can't be done, I understand. I already appreciate the unlimited image hosting