[rayiner]

The difference is ownership and control. The company owns its exchange server. It allows employees access under controlled conditions, for the employer's purposes. It's like leaving stuff in your garage that your lawn guy might have access to. But the company doesn't own Google's server. It has little control over how Google accesses that data. And Google accesses that data for its own purposes. That's more like leaving your stuff in someone else's garage.

Now, Google has a 4th amendment right with respect to "your" mail sitting on its servers, even though its employees have access to it. Because it's really "Google's mail," stored on Google's property, under Google's control.