coexistence

When you connect Exchange Server 2003 to another messaging system, including an earlier version of Exchange, the two systems coexist. A coexistence period can be short-term (enough time to migrate users from an existing messaging system to Exchange 2003), or it can be long-term (a permanent connection to the messaging system of another department that is not moving to Exchange 2003). [Exchange 2003]

In a coexistence scenario, any combination of the following versions of Microsoft Exchange is deployed in a single Exchange organization: Exchange 2000 Server, Exchange Server 2003, and Exchange Server 2007. These multiple versions of Exchange are able to communicate with each other and share data resources, recipient information, and configuration information. [Exchange 2007]