Error 401.3 when configuring RPC over HTTP on Exchange 2003 after installing Windows Server 2003 SP1

RPC over HTTP/S is a method for connecting Outlook 2003 clients to the corporate Exchange Server 2003 from the Internet or WAN, without the need to establish a VPN session to the corporate LAN and/or needing to open many ports on your corporate firewall. The only ports you'll need to open on your firewall are TCP 80 and, if using SSL, TCP 443.
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When installing and configuring RPC over HTTP/S, one of the first steps that needs to be followed is to install the RPC Proxy component on the Exchange server or on a different�dedicated server. After installing the RPC Proxy component one needs to perform some changes on the RPC virtual directory in IIS, and then to configure the default web site to use a Digital Certificate.
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After performing these steps you should�check the availability of the connection by connecting to the RPC virtual directory with Internet Explorer (by going the the https://servername/RPC url), logging on as one of the domain users, and waiting to receive a "normal" error message. The "normal" error message described below is supposed to be an indication that the RPC Proxy is indeed functioning correctly:
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HTTP Error 403.2 - Forbidden: Read access is denied.
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It seems that Windows Server 2003 Service Pack 1 (SP1) changes something in how the RPC Proxy mechanism is integrated with IIS. One of the visual signs for this change is the fact that when you now try to connect to the RPC virtual directory you cannot seem to be able to log on.
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When�you get the logon pop-up box and you enter the right username and password in the format of
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domain\username
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your credentials are denied by the system, and you receive the same logon pop-up box 3 times in a row. After the 3rd try (which all fail, no matter what username you're using, not even the administrator's credentials), you receive the following HTTP error:
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HTTP Error 401.3 - Unauthorized: Access is denied due to an ACL set on the requested resource.
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The RPC Proxy check, as described in KB 833401, fails.

It seems that this error message does not prevent you from successfully configuring RPC over HTTP/S, and if you follow the outlined procedures found in KB 833401 you will be able to connect your Outlook 2003 client to your Exchange Server 2003 server successfully.

Exchange Server 2003 RPC over HTTP Deployment Scenarios:
http://www.microsoft.com/technet/prodtechnol/exchange/2003/library/ex2k3rpc.mspx

How to configure RPC over HTTP on a single server in Exchange Server 2003:
http://support.microsoft.com/?kbid=833401

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