I recently had to check the capacity of our Exchange 2007 Server due to having delays of several hours for email sends and receives. I created the small PowerShell script attached to send 1000 emails through the server. Feel free to download and modify as required.
It turns out it was a bad Server 2003 update that caused the Exchange issue, http://support.microsoft.com/kb/2509553, which has a subsequent hot fix available here http://support.microsoft.com/kb/2669182
Its recommended to run this out of business hours or during a systems maintenance period. Download the script here