Have a notification sent to your email when your inbox is full
Almost everyone I know forwards their email to somewhere else (I forward mine to Gmail) which means that every once in a while we don't realize but our inbox gets full and we don't realize for a few days and by then we could have potentially missed a couple of important emails. One of my friends' email clients automatically notifies him when his inbox is 90% full. That would be a very helpful feature to implement.
With the coming move to Office 365, the idea of “inbox is full” will go away. The new quotas will start at 50 GB and are expected to increase later this year.