Understanding the Address Book Service
Applies to: Exchange Server 2010 SP3, Exchange Server 2010 SP2
In versions of Microsoft Exchange earlier than Exchange Server 2010, Exchange provided a referral service that told clients such as Outlook where they could find a server running the NSPI service. This referral usually pointed Outlook to a global catalog server. But some Outlook Anywhere connections would point Outlook back to the local server, and the NSPI calls would be proxied to a global catalog server.
Outlook expects to find this referral service on the same server that's used for mailbox access. In Exchange 2010, both mailbox access and directory access are handled by the Client Access server.
Directory Access on a Client Access Server
When Outlook contacts the Client Access server, two possible actions occur. If the user's mailbox is on an Exchange Server 2007 Mailbox server or an Exchange Server 2003 server, the directory request is referred to the user's mailbox server. If the user's mailbox is on an Exchange 2010 Mailbox server, then one of two actions happens.
If the user's mailbox is in the same site as the Client Access server, the request is referred to the Client Access server. If the user's mailbox is in a different site, the request is referred to a Client Access server in the remote site.
For Exchange 2010, the Client Access server hosts both the referral service and the NSPI endpoint. These two components are necessary for directory access to flow through the Client Access server.
If your Client Access server is installed on a domain controller, Outlook will communicate directly with the domain controller and will bypass the Client Access server.
Exchange 2007 Users
If a user who has a mailbox on a Microsoft Exchange Server 2007 Mailbox server queries the referral service, the Exchange 2010 Client Access server will refer the user to the Exchange 2007 Mailbox server. This is the same behavior that was experienced in a pure Exchange 2007 environment.
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