ISA 2006 Authentication over HTTP

I implemented different ISA 2006 Reverse Proxy servers in conjunction with Microsoft Exchange 2003 or Windows Exchange 2007.
Today I configured ISA 2006 with Exchange 2007. I configured the Reverse Proxy server as I did always. And the connection from outside the network works perfectly. On the internal Exchange server I configured Basic and Integrated Authentication on the OWA virtual directory. The problem is that internal users now automatically log in to their webmail box when entering the URL from the Exchange server.
This is not the desired configuration, because internal users should be able to open other people’s mailboxes by logging in as that user. The customer also has an ISA 2006 on the internal network for forwarding proxy purposes.
I decided to publish Exchange 2007 on the internal ISA 2006 server as well. The configuration should use Form Based Authentication (FBA) over HTTP. After configuring and trying the connection, the user can’t access the ISA logon page. In the logging you find that Authentication over HTTP isn’t allowed.

Error Code: 403 Forbidden. ISA Server is configured to block HTTP requests that require authentication. (12250)

This is a default setting in ISA 2006 which can be disable. To allow Authentication over HTTP go to the Listener configuration. Go to the Authentication tab and Select Advanced. In the next tab enable the option Allow client authentication over HTTP. This option enables the using FBA over HTTP.

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