Honestly, I like this service. While I agree with everyone's sentiments as to it addressing a problem but not fixing it, I believe it's a great alternative. There's not a "one size fits all" solution to email overload, and this allows a user another channel to express their availability.

I would only suggest allowing a user to set their own alert levels for each category-- my emails constantly go upwards of 100, 200, 300 unread at a time, but I still consider that manageable. Allow it to be customized and you have an interesting project that has some great potential.