Nice to see that there is innovation going on in email, still.
A few years ago I had some spare time and decided to "scratch an itch" by implementing my own "ideal" email system. My requirements were:
- Accessible via a web client, at least as a secondary interface.
- No "lock in" to a particular front end.
- Fast search and message categorisation.
I ended up by putting all of the messages in a PostgreSQL database, with full text indexing, and then writing an IMAP interface to it. I have "virtual folders" that are implemented by SQL queries, with new ones created in response to IMAP actions. So in my IMAP client (any IMAP client) I can create a folder called "Keywords/Beer" and it immediately contains all messages containing that keyword, or I can create a folder called "People/Fred" and drag one message from Fred into it, and it immediately contains all messages from (and maybe to) Fred.
It works reasonably well with ~100,000 messages. The weak part is IMAP, which I found to be a rather nasty protocol. It is GPLed and available at http://decimail.org/ - however, it has never had much external interest and has suffered from bit rot, so I don't really encourage anyone to try to use it unless very keen.