Wrobinson | Nov 10 2007 6:09PM GMT
There can only be one Exchange organization per forest. In this scenario, you can use one of the existing forests or establish a new one as a resource forest for Exchange.
If this is a Windows Server 2003 Active Directory environment thoughout, then you can use forest trusts but each forest would have to be in Windows 2003 Native mode. Otherwise, you will have to configure explicit external domain trusts for each domain.
You are likely to encounter some political infighting determining where Exchange should reside, what policies to put in place, how and by whom it will be managed.
Here are some good URLs to review: