heliotrope

You should have a peek at what my former coworker William is doing with Heliotrope. Pretty nice email client that supports threading with incredibly fast searching. It's not exactly what you're looking for and it's still pretty raw, you have to be familiar with the command line, but it has a lot of potential. https://github.com/wmorgan/heliotrope

Heliotrope is a personal email server. It provides all the functionality you want in a modern email client: - proper message threading - labels - fast, full-text search over all messages with a complete query language - support for signed and encrypted email - an extensible JSON-over-HTTP API Heliotrope is a backend service against which email clients / MUAs can be written. To use it, you must use a client. For an example client, see Turnsole: http://github.com/wmorgan/turnsole. WHY ANOTHER PROTOCOL? WHY NOT JUST USE IMAP? Because IMAP is terrible and you want all those features listed above. https://github.com/wmorgan/heliotrope

How is IMAP unreliable for backups? If you want to keep the tags, take a look at heliotrope[1], a personal email server with tag support that can sync Gmail accounts. [1]: https://github.com/wmorgan/heliotrope