> Why not use portable Thunderbird with GPG on an TrueCrypt encrypted USB > stick via Tor using POP3s/IMAPs and TLS/SSL'ed SMTP? >
That wasn't the original quesiton. The asker specifically said web-based, without (in my mind) any of the tricks which would let you do browser side decryption. If we're writing applications to do this, decrypting on the client end isn't really all that hard or unheard of.
