Want to stress test you Exchange 2007 and see how it will perform under heavy load ? Microsoft provides you the tools you’ll need.
Here you will find the tools JetStress and Load Generator
http://technet.microsoft.com/en-us/exchange/bb330849.aspx
And here is a document how to setup and use Load Generator
http://www.microsoft.com/downloads/details.aspx?FamilyID=c7534cb2-4234-41f1-b0b2-ed9f376b4cb2&DisplayLang=en