Easy to use eMail encryption for the masses?

I was just wondering about how easy to use eMail encryption for the masses can be achieved or how at least some progress towards that goal could be made. I was thinking along the lines of an SMTP / encryption / key management proxy that runs locally on each PC and encrypts every eMail in the background before sending it through a usual SMTP server. Usage would roughly be as follows: - software prompts user about which eMail addresses to use for encrypted messages upon first start - the software then generates a corresponding public / private key pair and uploads the public key automatically to public key servers - the software matches eMail addresses in the user's address book with public keys available on public key servers - the software runs as a proxy background service - the user sets her eMail client to use the proxy for outgoing eMail - when the user sends an eMail through her usual eMail client the proxy automatically encrypts the eMail if the message recipient has a public key (if he hasn't, an optional message could be added to the eMail telling about why it's important to encrypt digital communication) and subsequently sends the eMail through a range of public SMTP servers - optionally, the software could also work as a proxy for incoming eMail, which would allow for blocking or auto-answering non-encrypted / non-signed eMails Has something along those lines been tried before? Does such a software exist already? Would you think it's worth the effort?