Also the received vs read mail ratio.

I've noticed Yahoo Mail starts labeling some website email alert lists I'm signed up for start diverting to Spam unless I look at some of them every once in a while.

If people are asking for email alerts and then not opening the emails because they've read the news post or seen the comment to their entry or are using the E-mail My Comments option as a way to archive things they've said online, it can mess with that ratio, fast.