Following emails not sent?

I could probably research this out and figure it out on my own, but if someone else out there has already run into this I'd like to know if you have the answer. Normally, when you follow someone, a follower email is generated. However, these aren't being generated anymore when I follow someone with a newly created account (I had them follow some test accounts, and no notifications were generated - plus traffic from follows is way way down).

Anybody know what requirements must be met before these notifications are generated? If not, I'm working on an answer for my own question.