Part of a merger required free/busy sharing with other business units. These units previously had no visibility or communications possibilities (outside of Internet relayed e-mail) so this presented an interesting challenge.
Prior to Exchange 2010 a one or two way interdomain trust would have had to have been created. Thankfully I managed to circumvent this task with the assitance of Microsoft Federation Gateway (MFG). For the sake of clarity and complete understanding for anyone attempting to complete this task (I could find no end-to-end documentation online for our scenario) I have included all steps taken.
Microsoft Federated Gateway allows you to securely share information with other e-mail organizations across the Internet.
The domain consisted of Mailbox, CAS and Client Access servers, all running Exchange 2007 SP1.
The process can be broadly broken down into 3 steps:
1. Updating all Exchange 2007 boxes to Exchange 2007 SP3 (SP2 will also work). This is to facilititate the installation of an Exchange 2010 CAS.