Symptoms
Consider the following scenario: You use forms-based authentication in Microsoft Internet Security and Acceleration (ISA) Server 2004 or ISA Server 2006 to publish a Microsoft Outlook Web Access (OWA) server. You use a third-party OWA add-in program to manage e-mail attachments.In this scenario, when client computers use OWA and download e-mail attachments, the computers cannot download attachments.Additionally,you may receive an error message that resembles the following:

Internet Explorer cannot download file from server.
Internet Explorer was not able to open this Internet site. The requested site is
either unavailable or cannot be found. Please try again later.
Resolution
This problem occurs because the ISA Server 2004 or ISA Server 2006 forms-based authentication filter adds a Cache-Control: no-cache header to all responses for Active Server Pages (ASP) page requests except for known requests that are used by OWA for attachment downloads. The Cache-Control: no-cache header is added to ASP page requests to avoid session problems that occur with some non-Microsoft browsers.
When you use a third-party OWA add-in program that uses ASP pages for attachment downloads, the Cache-Control: no-cache header that is added to the ASP page response prevents Windows Internet Explorer from caching the attached file. Therefore, Internet Explorer cannot download or open the file. For more information about this problem, click the following article number to view the article in the Microsoft Knowledge Base:
316431?(http://support.microsoft.com/kb/316431/) Internet Explorer is unable to open Office documents from an SSL Web site