Update is available that supports publishing Microsoft Exchange Server 2007 behind Internet Security and Acceleration (ISA) Server 2006
There is an available hotfix to be installed on ISA Server 2006 for publishing Exchange Server 2007 RTM servers.
ISA Server 2006 includes support for publishing the pre-release version of Exchange 2007. This software update adds support for publishing the original release version of Exchange 2007.
Get it from here: http://support.microsoft.com/kb/925403/en-us