To provide the highest level of fault tolerance for your organization, make sure that you select the correct editions of Exchange and Windows. Although all editions of Exchange 2003 and Windows Server 2003 have been designed with high availability features, selecting the correct editions of each is an important step in maximizing the fault tolerance of your messaging system.

Selecting an edition of Exchange

You can select from two editions of Exchange 2003: Exchange Server 2003 Standard Edition and Exchange Server 2003 Enterprise Edition. The following table lists the key differences between these Exchange 2003 editions.

Key differences between Exchange 2003 Standard Edition and Exchange 2003 Enterprise Edition

Feature	Exchange 2003 Standard Edition	Exchange 2003 Enterprise Edition
Storage group support	1 storage group	4 storage groups
Number of databases per storage group	2 databases	5 databases
Total database size	16 gigabytes (GB)	Maximum 8 terabytes, limited only by hardware (see Note that follows)
Exchange Clustering	Not supported	Supported when running Windows Server 2003, Enterprise Edition or Windows Server 2003, Datacenter Edition
X.400 connector	Not included	Included

Note:
For timely backup and restore processes, 8 terabytes of messaging storage is beyond most organizations' requirements to meet SLAs.

For more information about the differences between Exchange 2003 Standard Edition and Exchange 2003 Enterprise Edition, see the Exchange Server 2003 Edition Comparison Web site.

For information that compares the features in Exchange Server 2003, Exchange 2000 Server, and Exchange Server 5.5, see the Exchange Server 2003 Features Comparison Web site.

Selecting an edition of Windows

You can select from three editions of Windows Server 2003: Windows Server 2003, Standard Edition; Windows Server 2003, Enterprise Edition; and Windows Server 2003, Datacenter Edition. If you want to use Windows Server 2003, Datacenter Edition, you must select a vendor that supports your Windows Server 2003 deployment scenario. For information about the Windows Datacenter High Availability Program, see the Windows Datacenter High Availability Program Web site.

Note:
Although you can run Exchange 2003 on Microsoft Windows® 2000 Server with Service Pack 3(SP3), there are many advantages to using Windows Server 2003. For information about the advantages to running Exchange 2003 on Windows Server 2003, see Better Together: Windows Server 2003 and Exchange Server 2003.

For information about selecting an edition of Windows Server 2003, see the Introducing the Windows Server 2003 Operating Systems Web site.

For information about the high availability features of Windows Server 2003, see the Maximizing Availability on the Windows Server 2003 Platform Web site.

When selecting a client application for your users, consider deploying Microsoft Office Outlook® 2003. Outlook 2003 supports client-side caching by using Cached Exchange Mode. When Outlook 2003 is used with Exchange 2003, you can configure Cached Exchange Mode to enable users to read e-mail messages or do other messaging tasks in low-bandwidth networks and in situations where network connectivity has been lost. Requests for information notifications from the Exchange server are eliminated on the user's Outlook client, allowing the user to work without interruption in low-bandwidth, high-latency networks. Moreover, Exchange 2003 and Outlook 2003 significantly improve client performance by reducing remote procedure calls (RPCs) and conversation between the Outlook client and the Exchange server. For more information about the performance advantages of using Outlook 2003 and Exchange 2003 together, see the Exchange Server 2003 Client Access Guide.

It is important that your server applications (for example, management scripts and antivirus software) are reliable. As a best practice, you should not run an application on a server until it has proven to be reliable in a test and pilot environment.

For information about laboratory testing and pilot deployments, see "Laboratory Testing and Pilot Deployments" in System-Level Fault Tolerant Measures.

For information about how to plan for unreliable applications, see "Operational Best Practices" in System-Level Fault Tolerant Measures.

Downtime for Windows and Exchange-based solutions can be the result of faulty device drivers, out-of-date software, or inadequate change control processes. To protect your Exchange 2003 organization against problems that hardware and software vendors have identified and corrected, keep your servers up-to-date with the latest software updates (such as hardware drivers) and firmware updates (such as basic input/output system [BIOS] updates). Most software application and hardware vendors have Web sites that provide software and firmware updates for their products.

Regarding your operating system updates, it is recommended that you regularly download the latest Windows Server 2003 software updates. Some Windows Server 2003 updates fix known problems or provide security enhancements. To download the latest Windows Server 2003 software updates, see the Microsoft Windows Update Web site.

However, before you deploy software and firmware updates in your production environment, make sure that you:

• Test the reliability of the updated software and firmware.
  
• Can back out of the updates if necessary.
  

Testing software and firmware updates in test environment

Backing out of software and firmware updates