Understanding DSProxy, DSReferral, DSAccess and the Categorizer:

We already aware that both AD and Exchange are highly integrated, the relationship b/w AD and exchange is more complex and often misunderstood.
Exchange server uses two services – Dsproxy and Dsaccess – to access the GC Global Catalog.

Dsproxy is an exchange specific service, whereas Dsaccess is a Windows service in turn used by the exchange server.

Dsproxy:

1. In general, Outlook 2000 clients and above can access the GC directly, but the old outlook client doesn’t.
2. To address the above lack of access, exchange server provides a proxy service called Dsproxy, acts as an intermediary b/w client and the GC.
3. Dsproxy works as a facilitator to allow outlook clients to access information within the AD through the Name Service Provider Interface (NSPI).

Dsreferral:

1. Since the dsproxy helps outlook client to reach the GC directly. Dsreferral also enhance or work to facilitate dsproxy.
2. Dsreferaal is used especially when exchange server not running on GC server. In such cases, DS Referral has the capability to update the Outlook 2000 client's MAPI profile with an appropriate GC server.

Dsaccess:

1. In general, the exchange server shares the GC functionality with the other AD services, It is important to reduce the impact of exchange server queries on GC. Dsaccess become solution for this.
2. Dsaccess implement a cache that stores recently accessed information for a configurable length of time. This cache reduces the number of direct query on GC drastically.

Role of the Categorizer:

The SMTP Categorizer is a component of Exchange that is used to submit mail messages to the proper destination. When a mail message is sent, the Categorizer queries the DSAccess component to locate an Active Directory server list, which is then directly queried for information that can be used to deliver the message. Problems with the Categorizer are often the cause of DNS or AD lookup issues. When troubleshooting mail-flow problems, please use message tracking in Exchange Server 2007 to find the course of a message. If the message stops at the Categorizer, it is often wise to start troubleshooting the issue from a directory access perspective.