Does anybody offer an encrypted-on-the-server email setup which allows easy key revocation? It's desirable (revoke a key you used on your iPhone when you sell i

Does anybody offer an encrypted-on-the-server email setup which allows easy key revocation? It's desirable (revoke a key you used on your iPhone when you sell it, or use a key a single time from an untrusted PC, etc.), but really inconvenient because each email would need to be encrypted for each key. And when you create a new key, if you want to read emails sent before the key's creation you'll need some way for the server to go back and reencrypt all the older emails, etc. Seems like a can of worms, but it'd be awesome if someone somehow made it convenient. I might just be wishing for magic, though. EDIT: 'Sup with the downvote? Explain what's wrong with my post.