If I build an Active Directory (AD) forest with multiple trees, can I create more than one Exchange Server organization in the forest?
No; Exchange supports only one Exchange organization per forest. Exchange stores the configuration data (e.g., the list of extant servers and connectors) in the configuration naming context (NC), and only one configuration NC exists for each forest. By contrast, one domain NC exists for each domain. As a result, unlike Exchange Server 5.5, Exchange Server 2003 and Exchange 2000 Server don't care about which domains the users or servers are located in, as long as they're in the same forest.