I assume someone has already mentioned bitmessage.org (even though it's not email) it could replace email someday as a secure alternative.
Unless it becomes possible to send and receive messages to non-bitmessage users I highly doubt it will gain much acceptance. There's too much network effect to overcome.
It is already possible to configure Thunderbird mail client to route mail through the bitmessage network ... it will become just another protocol layer option like POP, IMAP, SMTP, etc.
Is there a tutorial? If so, I would love to do this.