Copying inbound and outbound exchange me

I’ve been charged with finding a way to retain a copy of all incoming and outgoing mail from our Exchange Server (v 5.5, SP3).

We’ve had a problem with some contractors and employees collaborating on competitive buisiness plans using our company systems. We’ve determined that the best way to retain evidence if this occurs again would be to keep a copy of all incoming and outgoing messages (not just a log entry, we want the entire message). Is there any way to do this, and how?