E-Mail should be based on Git
E-Mail should be replaced/supplimented with a distributed, CAS/DAG/reftree system like Git. 'Message-ID' should be sha1 checksum, and 'References' header (implemented by most mail clients for tracking replies/threads) should be a parent ref, just like the way commits work.
There are all kinds of sane ways this could work. No doubt someone has thought of this already... What existing projects are out there for this idea?