Interesting concept, especially with the recent changes Google has made to GMail/Apps. The biggest benefit/downside that jumps out at me is the complexity required to run an email system this way - Cassandra clusters, LDAP, external MTAs, and web severs all need to be managed. Obviously every email system requires these components to run, but most full-blown systems handle the relationships between components on their own.<p>TL;DR - Looks great for large businesses or maybe as a platform for a startup, but not as a hobbyist project.
This really does appeal to me, but I wish it didn't rely on Cassandra. That's a heavy dependency if you would otherwise have no java in your stack which many mail servers probably wouldn't. It's certainly an interesting departure from flat files or standard databases though.
hmm. email is one of the things that scales extremely good (see dovecot's directors for a sophisticated but simple way). i don't know why you will need such a big hammer.