What web-based email services provide end-to-end encrypted & signed email messages, such as in PGP message format?  Is special software (e.g. Flash, Java, or another plugin) required for the service?Note: By end-to-end encryption, I'm talking about where the email messages themselves are encrypted and signed, not merely the HTTP communication layer from browser-to-server being encrypted with SSL.
What web-based email services provide end-to-end encrypted & signed email messages?
email;encryption;security;digital signature