Ask HN: product idea

Thoughts on this product idea? I got it when cleaning out my email inbox this morning (on Gmail).

10% of my emails are from friends (good), 30% from work (ok), and the rest are messages from companies (Amazon) and social networks (Facebook). Mailing lists get filtered out of my inbox into labels.

If we could do something with those message type emails, wouldn't that be nice?

Then again, perhaps filters are ok for this?

Also, does it make sense (at all) to build a product on top of Gmail? Because they can build any really valuable service themselves, later, and push you out of the market. (Same with Twitter, ...)