Exchange 5.5 PST files and how to manage

Hi, What I would like to be able to manage PST files that are located on employees desktops. What I want to be able to do is delete email that is over 30 days old. I will be using the MS utility to delete old mail from the server but it wont work for PST’s located on client machines. If anyone knows of a utility that can do this please let me know. My company is implementing a retention policy and we need to be able to control PST’s also. Thanks