You have a mixed Microsoft Exchange Server 2010 and Exchange Server 2003 environment.
The Exchange Server 2010 server has Update Rollup 3 for Exchange Server 2010 Service Pack 2 (SP2) installed.
An Exchange Server 2003 user creates a new Microsoft Outlook profile or runs the Test E-mail AutoConfiguration tool in Microsoft Office Outlook 2007 or Microsoft Outlook 2010.
Note To run the Test E-mail AutoConfiguration tool, hold down the Ctrl key, right-click the Outlook icon in the notification area on the right side of the taskbar, and then click Test E-mail AutoConfiguration.
In this scenario, the following event is logged in the Application log on the Exchange Server 2010 Client Access server:
Source: MSExchange Autodiscover Event ID: 1 Task Category: Web Level: Error Description: Unhandled Exception "User setting 'PreferredSite' is not available."
This issue occurs because the Exchange Server 2010 Client Access server does not calculate the PreferredSite value appropriately for the Autodiscover service request from the Exchange Server 2003 user.
To resolve this issue, install the following update rollup:
2785908 Description of Update Rollup 5 version 2 for Exchange Server 2010 Service Pack 2
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about Update Rollup 3 for Exchange Server 2010 SP2, click the following article number to view the article in the Microsoft Knowledge Base:
2685289 Description of Update Rollup 3 for Exchange Server 2010 Service Pack 2