well, you can certainly erase some PMs if your box is full. Just don't do what I did: erase a bunch of the precious received messages I stored on my computer to make space, until I realized a tad later that I had only a few left and the box was still almost full. It then dawned on me that what was filling the box was my *sent* messages which could not car less about....! Idiot! Should have erased those first!