Exchange Server Stress and Performance Tool
This tool simulates large numbers of client sessions running on Exchange Server 2007.
Get it here: http://www.microsoft.com/downloads/details.aspx?familyid=7f944850-945f-4e60-b6d6-cf7341d7f9c3&displaylang=en&tm
Ice Rocket : Exchange Server 2007, Performance, Stress, Tools