Event ID 1 is logged on the Exchange Server 2010 Client Access server in a mixed Exchange Server 2010 and Exchange Server 2003 environment.
Error Message: -
Unhandled Exception “User Setting PreferredSite is not available”
Problem Statement: -
1. When you have Exchange 2010 RU3 or less and Exchange 2003 in a mixed mode environment, you could see Event ID 1 keep logged on Exchange 2010 Client Access server.
2. 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.
Microsoft (Scott) confirmed that this issue with Microsoft software bug to resolve the issue you need to upgrade the Exchange 2010 RU3 to Exchange 2010 RU5v2.
Please follow the below Microsoft Technet Article for upgrade Exchange 2010 RU3 to Exchange 2010 RU5v2.