There are two parts of mail, obviously: the back-end store and forward system, and the client side. The client used to be simply a place to present messages, and the whole function was moving documents/files/messages back and forth. But now, the back-end deals with things such as spam filtering and authentication and scaling, and the user side has expanded to include calendar and contact management, increasingly integrated.
More fundamentally, 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.
Mail (which may change its name) becomes not just a collaboration tool, but also a personal workflow manager. It’s the inbox not just for messages, but for applications, for RSS feeds, for IMs and voice messages, and of course it helps manage (and integrate) SIP phone calls, too.
The real value of the new mail, though, will be attention management rather than content management. In an iterative process based on explicit user instructions and watching of user behavior, mail will start to know what you want to see now, what you want to see later (and when), and what you want to see never.