This subject was my first article that I've written here in the MSExchange.org, you can check out it at http://www.msexchange.org/tutorials/Transport-Rules-Exchange-Server-2007.html
The post of today in MsExchangeTeam talked about the same subject, as it follows:
Exchange Server 2007 provides an easy and flexible way to set rules for message routing and content restriction by Transport Rules. Transport Rules allow administrators to quickly apply corporate or compliance policies to e-mail messages that flow through an Exchange organization. Transport Rules can be managed with an Exchange Management Console Transport Rules Editor wizard or within the Exchange Management Shell (shell). The Transport Rules Editor wizard is designed to be very similar to the Outlook Rules Wizard, a familiar interface. This blog post introduces the Transport Rules Editor GUI and illustrates how to use it to accomplish an Ethical Firewall scenario.
Read more at source: http://msexchangeteam.com/archive/2006/12/12/431879.aspx