Karl Dubost pointed me to Paul Prescod's work on RESTmail, that fits the bill perfectly (at first glance). The only piece missing is the authentication bit for him, and we can solve that. So we should see if we can join forces there....
Blogging is like e-mail with access control. Post a blog entry that can be read only by the people you intend to read using access control. Ping them. They use WebID to authenticate and can read it, copying the entry over to their store.
Result: secure e-mail that arrives in a couple of connections at its destination.
Next: write a pop/smtp proxy, so that people can use their current e-mail clients to do their work.
Sending: When someone writes an email it is sent to the smtp server. If that server has a webid-mail endpoint to ping it use the RESTful mail. If they don't forward to a traditional smtp server
Receiving: the proxy receives all traditional and WebId Mail, and presents it to the client using a traditional SMTP service.
Additional feature: Create a nice Web Interface so that you get webmail for free.
Could also work with a WebID enabled RSS reader.
PS. I think I mentioned this before, but just talked about it at DERI, so it's fresh in my mind.