Most web browser clients for email are rather frustrating if you're used to a rich client such as Outlook or Notes. And so most business people use a rich client, most of the time.
Gradually, the browser experience is improving. The best web client for the last couple of years has been, and continues to be, Scalix Web Access--see https://www.scalix.com/products/scalixwebaccess.html. This is a very impressive product. In fact, it's hard to tell you're using a browser. Eg, it has its own set of pull-down menus; tips appear when you hover your mouse over icons; you can drag and drop messages into a folder hierarchy; response is fast (no waiting for page redraws by the server).
Rich clients have a very high administration overhead. So clearly, if a browser can delivery a high quality experience, it's far better to use than a rich client.
... David Ferris