Exchange 2007 supports a variety of Active Directory forest options, such as:

• No Forest   Active Directory is required for most Exchange 2007 deployments. However, there is an Exchange 2007 server role that does not require Active Directory. The Edge Transport Server Role in Exchange 2007 has been designed specifically to be operated outside of an Active Directory forest. The Edge Transport Server Role does not use Active Directory for transport and routing or for storing server configuration information. Instead, it uses Active Directory Application Mode (ADAM).
  
  
• Single Forest   In this topology, Exchange is installed in a single Active Directory forest that spans the whole organization. All user and group accounts and all the Exchange configuration information are located within the same forest.
  
  
• Resource Forest   This topology is one of two multiple forest topologies supported by Exchange 2007. In this topology, Exchange is installed in an Active Directory forest that does not contain the user and group accounts. This topology is typically deployed to facilitate separation of Active Directory and the Exchange administration space. In a resource forest, one forest is used for accounts and authentication, and a separate Exchange forest is used for Exchange. All Exchange and mailbox configuration data is contained in the Exchange resource forest.
  
  
• Cross-Forest   This topology is one of two multiple forest topologies supported by Exchange 2007. In this topology, Exchange is installed into multiple, different Active Directory forests. This topology is typically deployed in highly distributed organizations, where different groups want to retain management ownership of their individual space. In this topology, each forest has a complete Exchange deployment and a unique Exchange organization object. The separate Exchange systems are frequently configured to synchronize recipients between forests to provide a single global address list. They may also be configured to share other common messaging features, such as e-mail messages, free/busy data, and public folders by one or more connectors.
  
  

  Note:

  For the discussions in these topics, the cross-forest and resource forest topologies do not include federated environments. Federated environments exist between two or more disparate organizations where there is no common organization or relationship. In the earlier discussion of multi-forest topologies, some relationships exist between the different forests. The forests are part of a company, an organization, or some other common unit.

For more information about forest topologies for Exchange 2007, see Active Directory Forest Topologies.