My personal recommendation would be Sigaint, which you can access using the Tor Browser Bundle at the following .onion address:
http://sigaintevyh2rzvw.onion/
Regardless of which email provider you choose, you should be using PGP. Someone I respect once described email this way:
The authorities LOVE email -- and why not? From their perspective, it is a nearly-perfect information resource, as:
1) Emails are written by the suspects themselves, in their very own words;
2) Emails are date-and-time stamped, showing precisely when something was said; moreover, there is a complete chronological record of who said what to whom, and when.
3) Emails are, by their very nature, machine-readable -- this makes them searchable, archivable, and amenable to database entry. In short, they are in an ideal form to be processed for evidentiary purposes by the authorities.
4) Finally, as a general rule, emails are easy-to-harvest -- think GMail. Email inboxes can sometimes contain YEARS worth of email -- think YEARS worth of evidence -- all ready and waiting to be harvested by LEA.
So, from the point of view of the authorities: What's not to love?
So, what lessons can be learned from this?
Using Tor to access a web-based email account is the best solution, IMO. I say this because Tor ensures your anonymity -- the email provider does not have your info to give up to the authorities.
Use PGP -- preferably using software run on your own hardware, with keys stored ONLY on your own computer or flash-drive.
Learning to use PGP is the single best investment of your time that you can make -- it might even make the difference between going to jail or not. It's like training -- don't take shortcuts, invest the time to do it right.
Please remember: there is no service, anywhere, that you can pay for, that will provide security for you. When push comes to shove, they will roll over on you, because, at the end of the day,
no one is going to go to jail for you.
Hushmail is the cautionary tale here -- they promised that the authorities would not have access to the plaintext (unencrypted) emails; in the end, they broke that promise, and people ended up in jail.
Take your security into your own hands -- learn to do it for yourself.
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