Was just looking into how the PM limits fucntionality works. Turns out that if someone has hit their limit, any attempt to send them another PM will fail.
Seems to me that this is not a good thing. It could possibly lead to problems with important messages not being sent.
It'd be better if the message was sort of sent, but stacked in a queue that they can't read until they clear some space. This would be accompanied by a notice informing them that they had messages waiting, and needed to throw the garbage out.
I'm not sure how the system works in terms of informing ... I could see adding a weekly maintenance task that checks for all mailboxes that are 90% full and sending them a nag email that unless they clean up they will start missing messages. It might already do that, but I don't know.
Saving them in a queue will lead to madness, I'm sure of it ... so if its full I say tough fudge nuggets you loose, just like any email system.