Outlook Web Access prompts three times for credentials when connecting through proxy server

When you log on to a mailbox by using Outlook Web Access (OWA) through a proxy server, you may be prompted for credentials three times, and then a blank screen or an "Access is Denied" message may be displayed.

The default Web site on the proxy server has Windows NT Challenge/Response authentication enabled and Basic authentication disabled.

You need to enable Basic authentication and disable Windows NT Challenge/Response authentication at the default Web site on the proxy server. To do this, perform the following steps:

1. At the proxy server, start the Internet Service Manager tool.
2. Right-click the default Web site, and then click Properties.
3. Click the Directory Security tab, and then under Anonymous Access and Authentication Control, click Edit.
4. Under Authentication Methods, click to select the Basic Authentication check box, and then click to clear the Windows NT Challenge/Response check box.

After you enable Basic authentication and disable Windows NT Challenge/Response authentication, you need to stop and restart the World Wide Web services. To do this, perform the following steps:

1. Click the default Web site, and then click the small black square in the toolbar to stop the service.
2. After the service stops, click the small black triangle that points to the right to restart the service.

Microsoft Proxy Server version 2.0 implements the Web Proxy service by using an Internet Server API (ISAPI) filter that loads under the context of the default Web site. Because of this, the same considerations that apply to Outlook Web Access configuration also apply to the default Web site on a proxy server.

Windows NT Challenge/Response (NTLM) cannot authenticate users who do not have a direct connection to the Internet Information Service (IIS) server, so users who connect over the Internet are unable to authenticate by using NTLM.

For more information, click the following article numbers to view the articles in the Microsoft Knowledge Base: For more information about how to correctly configure Outlook Web Access, please see the Troubleshooting Guide for Outlook Web Access at the following Microsoft Web site: