Read “Streams: Mail 3.0 concept”

Streams are not created on a personal basis, they are shared between people interested in them. Streams are like shared folders for individual threads. This additional layer keeps your communications organized: you don’t anymore have one cluttered inbox, threads are now sorted in streams with authors’ hands. Streams are high-level enough to make inbox organization effortless and not to introduce significant overhead.

Interesting ideas about making email from an inbox to streams.