How the Autodiscover Service Works
When you install the Client Access server role on a computer that is running Exchange 2007, a new virtual directory named Autodiscover is created under the default Web site in Internet Information Services (IIS). This virtual directory handles Autodiscover service requests from Outlook 2007 clients and supported mobile devices in the following circumstances:
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When a new user account is configured or updated.
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When a user periodically checks for changes to the Exchange Web Services URLs.
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When underlying network connection changes occur in your Exchange messaging environment.
Additionally, a new Active Directory object named the service connection point (SCP) is created when you install the Client Access server role.
The SCP object contains the authoritative list of Autodiscover service URLs for the forest where Exchange 2007 is installed. For more information about SCP objects, see Publishing with Service Connection Points.
Figure 1 illustrates how a client connects to a Client Access server the first time from inside an internal network.
Figure 1 The Autodiscover service process for internal access.gif)
For external access, the client locates the Autodiscover service on the Internet by using the primary Simple Mail Transfer Protocol (SMTP) domain address from the user's e-mail address. Depending on whether you have configured the Autodiscover service on a separate site, the Autodiscover service URL will be either https://<smtp-address-domain>/autodiscover/autodiscover.xml or https://autodiscover.<smtp-address-domain>/autodiscover/autodiscover.xml. Figure 2 illustrates a simple topology with a client connecting from the Internet.
Figure 2 The Autodiscover service process for external access
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When the client connects to the Active Directory directory service, the client looks for the SCP object that was created during Setup. In deployments that include multiple Client Access servers, an Autodiscover SCP object is created for each Client Access server. The SCP object contains the ServiceBindingInfo attribute that has the fully qualified domain name (FQDN) of the Client Access server in the form of https://CAS01/autodiscover/autodiscover.xml, where CAS01 is the FQDN for the Client Access server. By using the user credentials, the Outlook 2007 client authenticates to Active Directory and searches for the Autodiscover SCP objects. After the client obtains and enumerates the instances of the Autodiscover service, the client connects to the first Client Access server in the enumerated list and obtains the profile information in the form of XML data that is required to connect to the user's mailbox and available Microsoft Exchange features.