email and attention management
More fundeamentally, as the world becomes more real-time and connected, the virtual and increasingly the actual configuration of the system is changing. There's a rich, complex, shared data store in the cloud, and mail is simply the passing of notifications and alerts that tell you to pay attention to/download specific items in the cloud that are new or changed or that someone wants to share with you. this creates huge challenges in version control, updating and permission management.
Outside the realm of person-to-person correspondence, email is about notification, information flow, and as Esther points out later in the post, attention management. Esther's right on the money that clients will evolve to help individuals manage that flow, and make sure that they're paying attention to the right stuff. But it's a two-way street -- the senders that get it right will win good will, incremental business and long term loyalty from their readers / users / customers.