When you upgrade to Exchange Server 2003, you first must make sure that your network and domain infrastructure is present and healthy. Exchange Server 2003 depends heavily on fast, reliable connections to Active Directory domain controllers, global catalog servers, and Domain Name System (DNS) servers. The installation and management components also depend on network basic input/output system (NetBIOS) name resolution.

Because of these requirements, the first step of any Exchange Server 2003 upgrade is an assessment of the Active Directory environment. You may have to perform at least a partial upgrade of your existing Windows NT Server 4.0 domains to Windows Server 2003 Active Directory, if you do not already have Active Directory deployed in your network.

There are two main methods to upgrade from Exchange Server 5.5 to Exchange Server 2003:

·         Standard deployment method

·         External upgrade method

The standard deployment method allows you to upgrade your Exchange organization by adding Exchange Server 2003 to your existing Exchange organization. You first perform an in-place upgrade of the Windows NT Server 4.0 domain to Windows Server 2003 Active Directory to prepare the user accounts in Active Directory. Then you join one or more new Exchange Server 2003 computers to your Exchange Server 5.5 organization and migrate your Exchange data to the Exchange Server 2003 computers. All the relevant information is migrated from the Exchange Server 5.5 directory service into Windows Server 2003 Active Directory.

The external upgrade method is commonly used to merge multiple Exchange organizations in organizations where nontechnical factors prevent the standard deployment method from being used. The external upgrade method is also used where technical factors, such as limited WAN bandwidth, does not allow use of the standard deployment method. You first create a new, empty Windows Server 2003 Active Directory environment and install your Exchange Server 2003 computers. You then perform a Windows NT Server 4.0 domain migration to move the user account data into your Windows Server 2003 Active Directory. You then migrate your Exchange Server 5.5 public folder replicas and mailbox data to your Exchange Server 2003 computers.

Exchange Server 2003 without SP1 lets you move user mailboxes and public folder replicas across administrative group boundaries only when the Exchange Server 2003 computer is in native mode. SP1 lets you perform these activities in mixed mode. You no longer are limited to performing an external upgrade if you have to move mailboxes between administrative groups. You can perform a standard deployment and have the flexibility to fine-tune your organizational structure as you proceed with your upgrade.