Definitions of users and acceptable performance vary from organization to organization. For example, a person who sends and receives a lot of e-mail may need a faster system than a user who sends mail infrequently. To help you determine the level of performance that's acceptable for your organization's users, you can use the Microsoft Exchange Server Load Simulator (Loadsim.exe).
Load Simulator can help you determine how many users your Microsoft Exchange Server computer can support. It is designed to provide a realistic load on a Microsoft Exchange Server computer by simulating the behavior of users on one or more Microsoft Exchange Client computers. It can be used for performance analysis and benchmarking or as a server stress tool.
Load Simulator takes advantage of multithreading, multiprocessing, and shared memory features in Windows NT Server to create and manage up to several hundred simulated users on a single client computer. Load Simulator makes it possible to run large server load simulations with a relatively small amount of hardware. It also provides a user interface for defining test topologies and user behavior and controlling simulations.