Exchange- Autodelete E-mail

Last night our Exchange server went south. Although we do not know what corrupted the database we know that we would be able to restore or fix the problem faster if the database was smaller.

A couple of my portfolio managers get about 100 email aday from brokers. What is the best way to Automatically delete email if it is lets say 20 days old?

I would like to do this from the server side. We are running exchange 5.5