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Part 2: How the DSProxy service works in Exchange Server and Active Directory

DSProxy service, one component responsible for Exchange Server's LDAP queries, helps alleviate communication issues that older and newer versions of Microsoft Outlook encounter.

The DSProxy service is a mechanism that helps make Exchange Server 2003 backward-compatible with Microsoft Outlook versions before Outlook 2000. Because Exchange 5.0 and Exchange 5.5. used their own built-in directory (the Exchange Directory) instead of storing information in Active Directory, older Microsoft Outlook clients can experience problems when run with Exchange 2003.

Versions prior to Outlook 2000 use MAPI to query the Exchange Directory for information about mailboxes in the organization. Since newer versions of Exchange Server do not contain an Exchange Directory, the DSProxy service acts as a directory emulator.

When older versions of Microsoft Outlook make MAPI calls directed toward the Exchange Directory, the DSProxy service intercepts the calls. It then reads the requested information from Active Directory and passes it back to the requesting client.

When the DSProxy service retrieves information from a global catalog server, it does not use LDAP queries, even though global catalog servers are in fact domain controllers. Instead, DSProxy uses the Name Service Provider Interface (NSPI) protocol for global catalog server communications.

NSPI is more efficient than LDAP, but is only supported by global catalog servers. Therefore, Exchange Server uses DSAccess to get the name of the global catalog server (via an LDAP query), and then uses NSPI to retrieve information from the global catalog.

For clients running newer versions of Microsoft Outlook, DSProxy acts as a referral service. This is important because the Global Address List (GAL) is derived from a global catalog server, and Microsoft Outlook does not contain a mechanism for contacting a global catalog server directly.

When Microsoft Outlook clients contact Exchange Server looking for a GAL, Exchange Server uses the DSProxy service to provide the client with a referral to a global catalog server. Once Microsoft Outlook receives this referral, it is able to communicate with the global catalog server directly.


 Home: Introduction
 Part 1: How the DSAccess service works in Exchange and Active Directory
 Part 2: How the DSProxy service works in Exchange and Active Directory
 Part 3: How the SMTP categorizer works in Exchange and Active Directory

Brien M. Posey, MCSE
Brien M. Posey, MCSE, is a Microsoft Most Valuable Professional for his work with Exchange Server, and has previously received Microsoft's MVP award for Windows Server and Internet Information Server (IIS). Brien has served as CIO for a nationwide chain of hospitals and was once responsible for the Department of Information Management at Fort Knox. As a freelance technical writer, Brien has written for Microsoft, TechTarget, CNET, ZDNet, MSD2D, Relevant Technologies and other technology companies. You can visit Brien's personal Web site at

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