Exchange 2003 uses administrative groups to organize Exchange objects for delegating permission to manage those objects. Exchange 2010 doesn't use administrative groups as a logical management unit for administrative delegation.

However, to support coexistence between Exchange 2003 and Exchange 2010, all Exchange 2010 servers are automatically put in a single administrative group when Exchange 2010 is installed. This administrative group is recognized in Exchange System Manager of earlier versions of Exchange as Exchange Administrative Group (FYDIBOHF23SPDLT).

Caution:

Don't move Exchange 2010 servers out of Exchange Administrative Group (FYDIBOHF23SPDLT) and don't rename Exchange Administrative Group (FYDIBOHF23SPDLT) by using a low-level directory editor. Exchange 2010 must use this administrative group for configuration data storage. Moving Exchange 2010 servers out of Exchange Administrative Group (FYDIBOHF23SPDLT) or the renaming of Exchange Administrative Group (FYDIBOHF23SPDLT) isn't supported.

You must use Exchange System Manager and utilities to manage the Exchange 2003 servers. In Exchange 2010, you must manage Exchange 2010 servers and mailboxes by using the EMC or the Shell. However, you can use the EMC to view some attributes on Exchange 2003 servers. For more information about EMC interoperability, see Exchange Management Console Interoperability.

When you're ready to upgrade a mixed mode environment, upgrade each Active Directory site individually. If you have Active Directory sites with only Exchange 2007 or Exchange 2003 in them, follow the instructions for upgrade from that version for that Active Directory site. For example, if you have Exchange 2007 in Active Directory site A, follow the upgrade instructions for Exchange 2007. If you have Exchange 2003 installed in Active Directory site B, follow the upgrade instructions for Exchange 2003. For more information about upgrading your Exchange 2003 and Exchange 2007 versions, see Understanding Upgrade to Exchange 2010.

If you have Active Directory sites with both Exchange 2003 and Exchange 2007 installed, follow the upgrade instructions from both Exchange 2003 and Exchange 2007, and perform the upgrade steps required by both. For more information about upgrading to Exchange 2010 in this scenario, see the following topics:

• Understanding Upgrade to Exchange 2010
  
  
• Install Exchange 2010 in a Mixed Exchange 2003 and Exchange 2007 Organization
  
  

As shown in the preceding figure, when you're upgrading your organization to Exchange 2010, you must begin with your servers in the Internet-accessible Active Directory sites, and then upgrade your internal Active Directory sites. This approach is necessary because Client Access server to Client Access server proxying is only supported from the newer Client Access server role versions (Exchange 2010) to the older Client Access server role versions (Exchange 2007) and not the reverse.

Within the first Active Directory site or sites you're upgrading, the first Exchange 2010 server role you install is the Client Access server role. We recommend that you upgrade a single Active Directory site at a time to Exchange 2010. Depending on the size of your Active Directory site, this might be a single Client Access server computer or a load-balanced array of Exchange 2010 Client Access server computers.

We recommend the following order when installing the Exchange 2010 server roles:

1. Client Access server role
   
   
2. Hub Transport server role
   
   
3. Unified Messaging (UM) server role
   
   
4. Mailbox server role
   
   

Note:
When upgrading to Exchange 2010, you can't perform an in-place server upgrade on an existing Exchange server.

For detailed information about upgrading server roles, see the following topics:

• Upgrade from Exchange 2003 Client Access
  
  
• Upgrade from Exchange 2003 Transport
  
  
• Upgrade from Exchange 2003 Mailbox
  
  
• Install Exchange 2010 in an Existing Exchange 2003 Organization
  
  

You can add the Unified Messaging server role later, or if you want to install it at the same time as the other server roles, you can do so by selecting Custom Exchange Server Installation.

Note:
You must deploy the Edge Transport server role in the perimeter network and outside the secure Active Directory forest.

The Client Access server role provides new features in addition to all the functionality provided by a front-end server in Exchange 2003. All client connectivity (including Microsoft Outlook MAPI connectivity) now goes through the Client Access server role. There are no longer any clients directly connecting to the Mailbox server role. The Client Access server role can coexist with Exchange 2003 servers. The following list describes the Exchange 2010 Client Access server dependencies and requirements for coexistence with Exchange 2003:

• Whether a user sees the Outlook Web App client of Exchange 2003 or the Outlook Web App client of Exchange 2010 depends on the location of the user's mailbox. For example, if the user's mailbox is located on an Exchange 2003 back-end server and the Client Access server is running Exchange 2010, the user will see Outlook Web Access, the Exchange 2003 client.
  
  
• The version of Microsoft Exchange ActiveSync that clients use depends on the server version hosting the user's mailbox. The user's mailbox must be located on a server running Exchange 2003 SP2 or Exchange 2010 to have Direct Push enabled for Exchange ActiveSync.
  
  
• When you perform an upgrade from Exchange 2003 to Exchange 2010, you typically upgrade all the Exchange servers in a specific routing group or Active Directory site to Exchange 2010 at the same time, configure coexistence, and then upgrade the next site.
  
  

Important:
When upgrading an Exchange 2003 organization, an Exchange 2003 front-end server is required to support the upgrade. For each Exchange 2010 Client Access server, you can only configure one Outlook Web Access 2003 URL for redirection. You can accomplish this with a single Exchange 2003 front-end server or a load-balanced array of Exchange 2003 front-end servers.

For more information about Client Access server coexistence between Exchange 2003 and Exchange 2010, and for information about new Exchange 2010 features, see Upgrade from Exchange 2003 Client Access.

Exchange 2007 introduced the Autodiscover and Availability services, and Exchange 2010 continues to rely on these services:

• The Autodiscover service configures client computers running Microsoft Outlook 2010, Outlook 2007, Entourage, and other client applications. The Autodiscover service can also configure supported mobile devices. The Autodiscover service provides access to Exchange features for Outlook 2010 clients connected to your Exchange messaging environment.
  
  
• The Availability service improves information workers' calendaring and meeting scheduling experience by providing secure, consistent, and up-to-date free and busy information to computers running Outlook 2007 or Outlook 2010.
  
  

For more information, see Understanding the Autodiscover Service and Understanding the Availability Service.

The Hub Transport server role is designed to handle all mail flow for the Exchange organization. It's also responsible for handling transport rules, journaling policies, and message delivery. This server is deployed in the Active Directory forest and is required for Exchange 2010 mailboxes to send and receive messages. Messages sent to the Internet are relayed by the Hub Transport server to the Edge Transport server or a third-party smart host.

You can add an Exchange 2010 Hub Transport server to an existing Exchange organization after you successfully deploy Exchange 2010 Client Access servers. When you introduce Exchange 2010 Hub Transport servers in your Exchange 2003 environment, all Exchange 2010 Hub Transport servers are placed in a single, separate routing group.

To enable mail flow between the Exchange 2010 deployment and your existing Exchange 2003 organization, you need to create a routing group connector. This routing group connector is created during the setup of your first Exchange 2010 Hub Transport server.

To learn more about introducing Exchange 2010 Hub Transport servers to your Exchange 2003 organization, see Upgrade from Exchange 2003 Transport.

For Exchange 2010 and Exchange 2003 Mailbox servers to coexist, you must be able to send mail among the mailboxes. Exchange 2010 uses the Hub Transport server to send mail. An Exchange 2010 Hub Transport server must be deployed in each Active Directory site that contains an Exchange 2010 Mailbox server. You also need a Client Access server in each Active Directory site where there's a Mailbox server. For more information about upgrading to an Exchange 2010 Mailbox server, see Upgrade from Exchange 2003 Mailbox.

If you move a mailbox from Exchange 2003 to Exchange 2010, and the mailbox is part of an e-mail address policy, the e-mail addresses for that mailbox are 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 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. For information about how to move mailboxes, see Managing Move Requests.

You can replicate public folder data between Exchange 2010 and Exchange 2003 public folder databases. To do this, you must create a replica of the public folder using the Exchange 2003 Exchange System Manager. For more information about Exchange 2010 and Exchange 2003 public folder coexistence, see Understanding Public Folders.

The Unified Messaging server role is designed to provide Unified Messaging for Exchange 2010 recipients. Unified Messaging combines voice messaging and e-mail messaging into one store that can be accessed from a telephone, a user's computer, or a mobile device. Users can access voice messages, e-mail, and calendar information located in their Exchange 2010 mailbox from e-mail clients, such as Outlook and Outlook Web App.

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 2010 Hub Transport server. For a recipient to use Unified Messaging, they must have an Exchange 2010 mailbox.

Versions of Exchange earlier than Exchange 2007 can't be upgraded and require you to deploy an Exchange 2010 organization with all the Exchange server roles, including Unified Messaging, and then move the Exchange 2003 (or earlier) mailboxes to an Exchange 2010 Mailbox server. For details, see Move Mailboxes from Exchange 2003 Servers to Exchange 2010 Servers.

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 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 don't have to perform any Active Directory preparation steps when you install the Edge Transport server.

If you're using the Exchange Intelligent Message Filter in Exchange 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 Exchange 2010 Edge Transport server is deployed to support an Exchange organization that hasn't yet deployed Exchange 2010, a limited set of features is available. You can't create an Edge Subscription in this scenario. Therefore, you can't use the Recipient Lookup or safelist aggregation features. For more information about Edge Transport servers and coexistence, see Upgrade from Exchange 2003 Transport.