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.
that should be a traditional POP or IMAP service of course.
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.