It has to be an all-in-one solution that you download that has to pretty much do everything for you so your mother can download a file and just go. With a simple wizard that lets you register a new e-mail address and potentially allows you to invite other users via their e-mail addresses... imported from Gmail or something like that.

I would not worry too much about compatibility with existing mail solutions or what not. It's a new service which is completely different from e-mail as far as the user is concerned. More advanced users can use it as e-mail if they want. Who would use it? Anyone who does not want all their mail to be a public spectacle.

Plenty of paranoid Americans out there right now that might be willing to give it a try.