If the box is hacked you wont be helped by encrypting your maildir, since the mail will have to leave the box decrypted. If you don’t add support for encryption in the client…
So you have to install Thunderbird with Enigmail or use Firefox with FireGPG if you want to use webmail. As far as I remember there is no other way, you have to get the sender to encrypt his mail. Because when you are hacked there is no way ou can encrypt you way out of that, sure you can protect the data already received, if you don’t support the encryption on the client.