prinnyex
Exploding Penguin
I was under the impression it was the confusingly-similarly-named notifications, given the talk about submissions and journals from net-cat's post. Seems a lot easier to get out of hand given it can easily balloon into thousands of them per user per month, and not many people clean them out.
notes are just as bad because even if you trashcan them, unless you go in the trashcan and delete them, they stay there forever. Same for outbox notes.