Publishing Exchange Server 2007 with ISA Server 2006
Found another nice article over on Technet.
Microsoft Internet Security and Acceleration (ISA) Server 2006 and Microsoft Exchange Server 2007 are designed to work closely together in your network to provide a secure messaging environment. This document explains how to publish computers running Exchange 2007 both to receive and send Internet e-mail messages and to allow clients to access their mailboxes from the Internet.