The metaphor of electronic mail never fully fit how people use e-mail. But, now, perhaps it might. Email could become a home for the kinds of communications that come in the mail: letters from actual people, bills, personalized advertisements, and periodicals.

While email's continued evolution is significant, what it has retained from the old web sets it apart from the other pretty, convenient apps. Email is an open, interoperable protocol. Someone can use Google's service, spin up a server of her own, or send messages through Microsoft's enterprise software. And yet all of these people can communicate... See more