Any organization that is transitioning from Exchange 2003 or Exchange 2000 to Exchange 2007 will experience a period of coexistence. A large organization that has many routing groups must plan their routing topology to maintain mail flow during the coexistence period.

Successful coexistence of Exchange 2007 with Exchange Server 2003 or Exchange 2000 Server depends on correct configuration of routing group connectors between Exchange 2007 Hub Transport servers and Exchange Server 2003 or Exchange 2000 Server bridgehead servers. When you install Exchange 2007 in a forest with an existing Exchange organization, you select to join Exchange 2007 to that organization. During installation, you are also prompted to specify an Exchange 2003 or Exchange 2000 bridgehead server in the routing group to which you want to create the initial routing group connector. Because of differences in how server-to-server communication occurs, you must configure routing group connectors to let the two Exchange versions communicate and exchange messages. For more information about how to plan a routing topology for an Exchange organization that is operating more than one version of Exchange, see Message Routing in a Coexistence Environment.

The Exchange 2007 server role features that are available to clients in the Exchange organization during the coexistence period depend on the version of the Exchange server where the user's mailbox is stored and the version of the e-mail client application that is used to access Exchange. To determine the order in which Exchange 2007 server roles are introduced to the organization, see Best Practices for Transitioning an Exchange Organization.

Exchange Server 2003 and Exchange 2000 Server used Simple Mail Transfer Protocol (SMTP) as the primary communication protocol between Exchange servers. In Exchange 2007, the server roles use RPC for server-to-server communication and rely on the Hub Transport server to perform SMTP transport.

Note:
For more information about how server-to-server communication occurs in Exchange 2007, see Transport Architecture.

Exchange Server 2003 and Exchange 2000 Server use Administrative Groups to organize Exchange objects for the purposes of delegating permission to manage those objects. Exchange 2007 does not use Administrative Groups as a logical management unit for administrative delegation. However, to support coexistence between Exchange Server 2003 or Exchange 2000 Server and Exchange 2007, all Exchange 2007 servers are automatically put in a single Administrative Group when Exchange 2007 is installed. This Administrative Group is recognized in the Exchange System Manager of earlier versions of Exchange as Exchange Administrative Group (FYDIBOHF23SPDLT). Administrative groups are not displayed in the Exchange 2007 Exchange Management Console.

Caution:

Do not move Exchange 2007 servers out of Exchange Administrative Group (FYDIBOHF23SPDLT) and do not rename Exchange Administrative Group (FYDIBOHF23SPDLT) by using a low-level directory editor. Exchange 2007 must use this administrative group for configuration data storage. We do not support moving Exchange 2007 servers out of Exchange Administrative Group (FYDIBOHF23SPDLT) or renaming of Exchange Administrative Group (FYDIBOHF23SPDLT).

You must use the Exchange Server 2003 or Exchange 2000 Server Exchange System Manager and utilities to manage the Exchange Server 2003 or Exchange 2000 Server servers. You can also use the Exchange tasks in Active Directory Users and Computers to manage Exchange Server 2003 or Exchange 2000 Server mailboxes. In Exchange 2007, you must manage Exchange 2007 servers and mailboxes by using the Exchange Management Console or the Exchange Management Shell.

Global settings that are configured for the Exchange Server 2003 or Exchange 2000 Server organization are stored in the Active Directory directory service. Exchange 2007 also applies those settings. After Exchange 2007 is introduced to the Exchange organization, all organizational-level settings must be maintained by using Exchange 2007 administration tools. For more information about how to transfer organizational settings from Exchange Server 2003 to Exchange 2007, see Managing Exchange 2003 Settings in a Coexistence Environment.

When you plan for a period of coexistence with Exchange 2007 and Exchange Server 2003 or Exchange 2000 Server, you must understand the differences in how each version determines its routing topology.

Exchange Server 2003 and Exchange 2000 Server use routing groups to define an Exchange-specific routing topology. Typically, routing groups are used to specify a set of well-connected Exchange servers. Servers in the same routing group can communicate with each other without the use of connectors. Ideally, the routing groups that are defined in your existing environment are based on IP subnets and closely mirror the Active Directory Site configuration. When more than one routing group is defined in an Exchange Server 2003 organization, the administrator must manually create routing group connectors to enable mail flow between Exchange Server 2003 servers that are in different routing groups. The routing group connector must specify a source server and a target server as the connector end points. A routing group connector defines a one-way connection, and a reciprocal connector must be created to establish mail flow in both directions. The source and target servers are the bridgehead servers for the routing group. The bridgehead servers relay e-mail to other routing groups on behalf of other servers in their routing group and receive e-mail from other routing groups for delivery to other servers in their routing group. Every Exchange 2003 or Exchange 2000 routing group should have at least one connector to another routing group before you introduce the first Exchange 2007 server. Event ID 5006 is logged for each Microsoft Exchange Message Database (MDB) that is located in a routing group that does not have a routing group connector path from the Exchange 2007 routing group. For more information about the Exchange Server 2003 routing topology, see Exchange Server Transport and Routing Guide.

Exchange 2007 uses the existing Active Directory site topology to define its routing topology. Servers that are located in the same Active Directory site can communicate without the use of connectors. E-mail that is routed to Exchange servers that are located in different sites must be relayed by Hub Transport servers. Hub Transport servers send e-mail to Hub Transport servers in remote sites by using the intra-organization Send connector. The intra-organization Send connector is an implicit connector that is computed by using Active Directory site and IP site link information. In Exchange 2007, an administrator no longer has to define an Exchange-specific routing configuration. However, an administrator can make Exchange-specific configuration changes to Active Directory sites and IP site link costs to control mail flow.

To support coexistence between these two routing topologies, all Exchange 2007 servers are automatically added to a single routing group when Exchange 2007 is installed. The Exchange 2007 routing group is recognized in Exchange System Manager in Exchange Server 2003 or Exchange 2000 Server as Exchange Routing Group (DWBGZMFD01QNBJR) within Exchange Administrative Group (FYDIBOHF23SPDLT). When the first Exchange 2007 Hub Transport server is installed in an existing Exchange organization, you must specify an Exchange 2003 bridgehead server to which to establish the first routing group connector. We recommend that you select a bridgehead server that is located in a hub routing group or in a routing group that has many mailboxes. The routing group connector links the routing group where the Exchange 2003 server resides and the Exchange 2007 routing group. The Exchange 2007 routing group includes all Exchange 2007 servers, regardless of the Active Directory site in which they reside.

Caution:

Do not move Exchange 2007 servers out of Exchange Routing Group (DWBGZMFD01QNBJR) and do not rename Exchange Routing Group (DWBGZMFD01QNBJR) by using a low-level directory editor. Exchange 2007 must use this routing group for communication with earlier versions of Exchange. We do not support moving Exchange 2007 servers out of Exchange Routing Group (DWBGZMFD01QNBJR) or renaming of Exchange Routing Group (DWBGZMFD01QNBJR).

The Hub Transport server that you are installing and the Exchange 2003 bridgehead that you select are configured as the source and target servers on two reciprocal routing group connectors. The selected bridgehead server is automatically added to the membership of the ExchangeLegacyInterop universal security group and is granted the permissions that are needed to send e-mail to and receive e-mail from Exchange 2007. This routing group connector creates a single connection point between Exchange 2003 and Exchange 2007. The bridgehead server that you select depends on your current routing environment. You can modify the list of source and target servers by using the Set-RoutingGroupConnector cmdlet in the Exchange Management Shell. It is a best practice to specify more than one source server and more than one target server to provide redundancy and server availability.

If your existing Exchange environment includes more than one routing group, you may want to create additional connection points between Exchange 2003 and Exchange 2007 to optimize mail flow. To create additional connection points, you follow these steps: 

• Determine how you will transition the organization to Exchange 2007. The order in which you decommission routing groups will determine which Exchange 2003 routing groups should connect directly with Exchange 2007.
  
• Modify the registry to suppress minor link state updates on all the Exchange 2003 and Exchange 2000 servers. This configuration change prevents connector state messages from being relayed throughout the organization by using link state updates, but does not prevent configuration change messages from being relayed. For more information, see How to Suppress Link State Updates.
  
• Use the New-RoutingGroupConnector cmdlet in the Exchange Management Shell to create all routing group connectors that specify Exchange 2007 Hub Transport servers as source or target servers. Configure a routing group connector from the Exchange Routing Group (DWBGZMFD01QNBJR) to each Exchange Server 2003 or Exchange 2000 Server routing group with which Exchange 2007 will communicate directly, and configure the corresponding reciprocal routing group connectors. You can use the Bidirectional parameter with the New-RoutingGroupConnector cmdlet to create both connectors in a single operation. These connectors will enable mail flow between Exchange Server 2003 or Exchange 2000 Server and Exchange 2007, and from Exchange 2007 to Exchange Server 2003 or Exchange 2000 Server. You must use the Exchange Management Shell cmdlets to create and configure routing group connectors between Exchange 2007 and earlier versions of Exchange. When you use the Exchange 2007 tools, the specified legacy Exchange servers are automatically added to the membership of the ExchangeLegacyInterop universal security group and the permissions that are required to allow a legacy Exchange server to send mail to and receive mail from an Exchange 2007 Hub Transport server are automatically granted. If you use Exchange System Manager to create a routing group connector between the Exchange 2007 routing group and any Exchange 2003 or Exchange 2000 routing group, this group membership is not updated correctly and the connector will not work correctly.
  

  Important:
  Placing Exchange 2007 servers and Exchange 2003 or Exchange 2000 servers in the same routing group is not supported.

  Important:
  If you deploy Exchange 2007 as a new organization, you cannot later install Exchange 2003 or Exchange 2000 in the Exchange 2007 organization. This is not a supported scenario. If you anticipate requiring Exchange 2003 functionality in your organization in the future, you must first install an Exchange 2003 organization and maintain at least one Exchange 2003 server.

For more information about how Exchange 2007 determines its routing topology, see Planning to Use Active Directory Sites for Routing Mail. For more information about the differences in routing behavior between Exchange 2007 and earlier versions of Exchange Server, see Message Routing in a Coexistence Environment.

Link State Routing

Send Connectors and Receive Connectors

X-EXCH50 Data

Message Tracking

The Edge Transport server role is designed to provide improved antivirus and anti-spam protection for the Exchange organization. The Edge Transport server also applies policies to messages in transport between organizations. This server role is deployed in the perimeter network and outside the Active Directory forest. The Edge Transport server can be deployed as a smart host and SMTP-relay server for an existing Exchange Server 2003 organization.

You can add an Edge Transport server to an existing Exchange organization without upgrading the internal Exchange servers or making any organizational changes. You do not have to perform any Active Directory preparation steps when you install the Edge Transport server. If you are using the Exchange Intelligent Message Filter in Exchange Server 2003 to perform anti-spam tasks, you can use the Edge Transport server to provide an additional layer of anti-spam protection. The Edge Transport server provides antivirus and anti-spam protection as messages enter the network.

When an Edge Transport server is deployed to support an Exchange organization that has not yet deployed Exchange 2007, a limited set of features are available. You can't create an Edge Subscription in this scenario. Therefore, you cannot use the Recipient Lookup or safelist aggregation features.

The Mailbox server role can coexist with Exchange Server 2003 and Microsoft Exchange 2000 Server mailbox servers. For Exchange 2007 and Exchange 2000 Server or Exchange Server 2003 mailbox servers to coexist, you must be able to send mail among the mailboxes. Exchange 2007 uses the Hub Transport server to send mail. A Hub Transport server must be deployed in each Active Directory site that contains an Exchange 2007 Mailbox server.

You can move mailboxes among Exchange 2000 Server, Exchange Server 2003, and Exchange 2007. For information about how to move mailboxes, see Moving Mailboxes.

If you move a mailbox from Exchange 2000 Server or Exchange Server 2003 to Exchange 2007, and the mailbox is part of an e-mail address policy, the e-mail addresses for that mailbox is automatically updated based on the configuration of the e-mail address policy. If the mailbox had a primary SMTP address that differs from the e-mail address that is enforced by the e-mail address policy, that SMTP address becomes a secondary SMTP address and the e-mail address generated by the e-mail address policy becomes the primary SMTP address.

This behavior differs from Exchange 2003 and Exchange 2000. In Exchange 2003 and Exchange 2000 , the e-mail address policy is not applied to a mailbox when it is moved. To prevent accidentally changing the primary SMTP address of a mailbox in an Exchange 2007 environment, you must configure the mailbox so that is does not automatically update e-mail addresses based on e-mail address policy. 

To configure Exchange 2003 or Exchange 2000 mailboxes, in Active Directory Users and Computers, right-click the recipient, and select Properties. On the E-mail Addresses tab, clear the Automatically update e-mail addresses based on e-mail address policy check box.

You can replicate public folder data between Exchange 2007 and Exchange 2000 Server or Exchange Server 2003 public folder databases. To do this, you must create a replica of the public folder using either the Exchange 2000 Server or Exchange Server 2003 versions of Exchange System Manager. The routing group connector between Exchange 2007 and Exchange 2003 can be configured to enable public folder referrals.

The Client Access server role provides the functionality that was provided by a front-end server in Exchange 2003. You must deploy the Client Access server role if you have e-mail clients that access Exchange by using Office Outlook Web Access, IMAP4, POP3, or mobile devices. The Client Access server role can coexist with Exchange Server 2003 and Exchange 2000 Server servers. The following list describes the Exchange 2007 dependencies and requirements for coexistence with Exchange Server 2003 and Exchange 2000 Server.

• Whether a user sees the Outlook Web Access client of Exchange Server 2003 or Exchange 2007 depends on the location of the user's mailbox. For example, if the user's mailbox is located on an Exchange Server 2003 back-end server and the Client Access server is running Exchange 2007, the user will see the Exchange Server 2003 version of Outlook Web Access.
  

  Important:

  Before you configure Client Access servers and decommission your Exchange Server 2003 front-end servers, determine whether you want to retain any Outlook Web Access settings or custom configurations, security updates, themes, and customization configurations, from your Exchange Server 2003 front-end servers. Installation of Exchange 2007 requires installation on new hardware, and no settings or custom configurations from Exchange Server 2003 are retained. Therefore, before you decommission your front-end servers and install Client Access servers, make sure that the Outlook Web Access settings and custom configurations on your Exchange Server 2003 back-end servers match the configurations on your Exchange Server 2003 front-end servers.

• The version of Exchange ActiveSync that clients use also depends on the server version that is hosting the user's mailbox. The user's mailbox must be located on a server that is running Exchange Server 2003 Service Pack 2 or Exchange 2007 to have Direct Push enabled for Exchange ActiveSync. A user whose mailbox is located on an earlier server version will be unable to use new features, such as global address list lookup and Exchange Search.
  
• When you perform a transition from Exchange Server 2003 to Exchange 2007, you will typically transition all the Exchange servers in a particular routing group or Active Directory site to Exchange 2007 at the same time, configure coexistence, and then transition the next site. If you are installing the server roles on separate hardware, we recommend that you deploy the server roles in the following order:
  
  1. First, install the Client Access server role to replace all front-end servers.
     
  2. Deploy the Hub Transport server role and configure routing group connectors, Send connectors, and Receive connectors.
     
  3. Deploy the Mailbox server role and move user mailboxes to the new server.
     

     Important:
     Using an Exchange Server 2003 front-end server together with an Exchange 2007 Mailbox server is not supported.

• The Outlook Web Access URL used to access the logon dialog depends whether the user's mailbox is located on an Exchange 2003 back-end server or on an Exchange 2007 Mailbox server. Use one of the following URLs to reach Outlook Web Access:
  
  • If the mailbox is located on an Exchange Server 2003 back-end server, use http://<servername>/Exchange.
    
  • If the mailbox is located on an Exchange 2007 Mailbox server, use either http://<servername>/owa or http://<servername>/Exchange.
    

The Unified Messaging server role is designed to provide Unified Messaging (UM) for Exchange 2007 recipients. UM combines voice messaging, fax, and e-mail messaging into one store that can be accessed from a telephone, a user's computer, and a mobile device. Users can access voice messages, e-mail, fax messages, and calendar information that are located in their Exchange 2007 mailbox from e-mail clients, such as Outlook and Outlook Web Access.

The Unified Messaging server is new to Exchange, and it cannot interoperate with earlier versions of Exchange Server. The Unified Messaging server depends on the Hub Transport server and Mailbox server. All SMTP mail submitted from a Unified Messaging server must be submitted to an Exchange 2007 Hub Transport server. For a recipient to use Unified Messaging, they must have an Exchange 2007 mailbox.

For more information, see the following topics:

• Message Routing in a Coexistence Environment
  
• Planning to Use Active Directory Sites for Routing Mail
  
• Upgrading to Exchange 2007
  
• Best Practices for Transitioning an Exchange Organization
  
• Coexisting with Exchange Server 2003 and Exchange 2000 Server
  
• How to Install Exchange 2007 in an Existing Exchange Server 2003 Organization
  
• Managing Exchange 2003 Settings in a Coexistence Environment
  
• How to Suppress Link State Updates
  
• How to Create Routing Group Connectors from Exchange 2007 to Exchange Server 2003