In Exchange 2007, we introduced transport rules – a powerful feature that allows you to inspect different parts of a message such as sender, recipient, subject and headers and take actions like rejecting a message, deleting it, redirecting it to another recipient, adding a message header or a disclaimer. In previous versions of Exchange, you would need to write a transport event sink to accomplish similar things. With an easy-to-use interface in the EMC, transport rules make such tasks as easy as creating Inbox rules (created by users using Outlook/OWA). Command-line jockeys can create and manage transport rules using *-TransportRule cmdlets from the Shell. More about transport rules in Understanding Transport Rules.
In Exchange 2010, we added a number of new predicates (which are used to create conditions and exceptions), including the ability to inspect attachment content and predicates to evaluate Active Directory attributes of the sender or recipients. A complete list of predicates can be found in Transport Rule Predicates.