Applies to: Exchange Server 2007 SP2, Exchange Server 2007 SP1, Exchange Server 2007 Topic Last Modified: 2006-11-22

You cannot upgrade an existing Microsoft Exchange Server version 5.5 organization to Exchange Server 2007. You must first migrate from the Exchange Server 5.5 organization to an Exchange Server 2003 or an Exchange 2000 Server organization. Then you can transition the Exchange 2003 or Exchange 2000 organization to Exchange 2007. 

For more information about migrating from Exchange Server 5.5 to Exchange 2003, see Migrating from Exchange Server 5.5 to Exchange Server 2003 in the Exchange Server Deployment Guide.

For more information about migrating from Exchange Server 5.5 to Exchange 2000, see Upgrading from Exchange Server 5.5 to Exchange 2000 Server: A Six-Step Case Scenario.

For more information about transitioning from Exchange 2003 or Exchange 2000 to Exchange 2007, see the following topics:

• How to Transition from Single Forest to Single Forest
  
• How to Transition from Single Forest to Cross-Forest
  
• How to Transition from Single Forest to Resource Forest
  
• How to Transition from Cross-Forest to Cross-Forest
  
• How to Transition from Resource Forest to Resource Forest
  
• Coexisting with Exchange Server 2003 and Exchange 2000 Server