User notifications.
 

Every now and then I get complaints from users that the PM box is full for somebody they're trying to send something to.

Is there a way, or does a mod exist, that will cause a pop up message to appear to anyone who has a full PM inbox?

It looks like the variable $show['pmwarning'] is set to true if a user's pm box is 90% full or more, but I don't see anywhere it's used (I haven't actually tested to see if any warning appears anywhere). But I suppose you could check that in a template condition and display whatever you wanted.

Thanks for the answer. Unfortunately I wouldn't even know where to start to do what you suggested.

I was sort of hoping some clever bugger had already written a mod to do it.