·         Many Exchange Server 5.5 deployments have multiple Microsoft Windows NT® Server 4.0 domains. In these deployments, you must carefully manage domain trust relationships and permissions. In Exchange Server 2003, the Exchange organization covers the whole Active Directory forest and easily spans multiple domains. All recipient and mailbox information is stored in Microsoft Windows Server™ 2003 Active Directory. This information is automatically replicated with user account information. Mail-enabled domain groups and contacts are defined in Active Directory and are visible throughout the forest by using the global address list (GAL).

·         Improved deployment flexibility   Exchange Server 5.5 restricts moving and renaming objects in the organization because configuration information is tied to the Exchange Server 5.5 directory service on a particular server. In Exchange Server 2003, the configuration information is tied to the user account or Exchange server objects in Windows Server 2003 Active Directory. You can easily rename recipients and move mailboxes and public folder replicas between servers in the organization without breaking group or distribution list memberships. With Exchange Server 2003 Service Pack 1 (SP1), you can move mailboxes across administrative groups, which are similar to Exchange Server 5.5 sites.

·         Improved database architecture   Exchange Server 5.5 provides a single set of message databases. You must partition your users and data at the server level. With Exchange Server 2003, you have up to four storage groups for fine-grained management of your backup Windows and mailbox store policies. By using a recovery storage group, you can immediately recover basic messaging functions while you restore backup content. Your existing Exchange mailboxes are automatically rebuilt when they are migrated to a server running Exchange Server 2003. They also are defragmented. Defragmentation frequently resolves lingering issues of database corruption.

Storage options   In Exchange Server 5.5, mailbox databases must be located on locally attached volumes, such as a direct access storage device (DASD) or a shared Storage Area Network (SAN). With Exchange Server 2003, you have more flexible storage options. You can use certified Internet SCSI (iSCSI) solutions and host your mailbox databases on Windows Storage Server certified network-attached storage solutions. This article does not cover storage configuration in detail. For more information about storage configuration, see Microsoft Knowledge Base article 839687, "Microsoft support policy on the use of network-attached storage devices with Exchange Server 2003"