After two and a half years of work: Pipemail sees public light. Pipemail encrypts emails using aes256; but any symmetric-key encryption cipher works, really. It's built around the idea that an email provider should never be able to retrieve the emails that their users send. To achieve this I invented a fragmented key assembling method.
There's some other nifty features available, like that we automatically strip EXIF tags from attached image files and allow users to send emails that are only readable once by the recipient. You can check out the documentation here: http://bit.ly/fIjLPk
I really hope that you like it. Hopefully we can shed light in this modern Big Brother day and age.
Oh, and it's free of course. But you can donate if you like it :-)
Review my startup: Secure webmail service
pipemail.org