You can build client functionality in Microsoft Exchange Server by extending the Microsoft Exchange Client or by developing your own client application.
Extending the Microsoft Exchange Client usually involves adding functionality to the Viewer, such as a new pull-down menu or function key. The Viewer is where users access, organize, and exchange information, as shown in the following figure.
The Viewer groups many types of information in one place, including such diverse items as e-mail, word processing, voice mail, faxes, spreadsheets, presentation graphics, meeting requests, and customer contact reports. The toolbar is the interface for tasks such as printing, creating a new message, deleting an item, or viewing the address book. This interface is consistent across all platforms supported by Microsoft Exchange Server.