What is the best e-mail system to preserve privacy?

[sir pball]

4. Tormail is good for anonymity

Tor Mail's goal is to provide completely anonymous and private communications to anyone who needs it.[1] The service provider owners declare they are anonymous and cannot be forced to reveal anything about a Tor Mail user.
Tor Mail service consists of several servers, this hidden service, and an incoming and outgoing internet facing mail servers - the relays, they relay mail in and out of the Tor network, they are disposable servers purchased anonymously and not traceable to user or service provider.
The owners declare the only things stored on the hard drive of those servers is the Exim mail server and the Tor software. No emails, logs or personal data are stored on those servers, thus it doesn't matter if they are seized or shut down. They claim to be prepared to quickly replace any relay that is taken offline for any reason.

I don't have time right now to explain the underlying Tor network (work awaits), but the Wikipedia article will get you started. It sounds much more complicated than it really is; you just download the Tor Browser and go.