When using Outlook Web Access (OWA), users are prompted to login after clicking on links.

If you are prompted for credentials when you click a hyperlink to an external Web site, follow these steps:

1. In Internet Services Manager, view the properties of the “Exchweb” virtual directory.
2. On the Directory Security tab, click the Edit button under anonymous access and authentication control.
3. Click to select the Basic Authentication check box, include the default domain name, and then click OK.
4. When prompted to select which “child nodes” to include, click the bin node.

http://support.microsoft.com/default.aspx?scid=kb;en-us;817312&Product=exch2003