I was mildly irritated to come back to my computer and find an automatic message from my ISP that I was over my quota. The reason I was over my quota is that some file sucked up all my space. It isn't the first time that I have found a multimegabyt file called 'core'. As soon as I deleted it, I got another automatic message saying I was back under quota.
If anyone, unlikely as it may seem, tried to send me an email and got a message bounced, this is why. Resend and it will be okay.
Edited to add:
I just heard back from my ISP -- he's increased my quota. The automatic notification is new, and now he will see about setting a script to check on 'core' files to keep this sort of thing from happening again.