If you are looking to roll out Windows Server 2003 SP1 on your Exchange servers and you are doing clustering on back-end servers, we have a hotfix that you will need to install.
From: 841561 "500 - Internal server error" error message when a user tries to access a clustered Exchange Server 2003 back-end server by using Outlook Web Access
“This problem is caused by some of the security enhancements that are included with Windows Server 2003 SP1. In this scenario, when a user tries to access a mailbox by using Outlook Web Access, HTTP requests into the clustering API by impersonating the logged-on user. However, there have been security changes in Windows Server 2003 SP1. The security restrictions for the APIs that perform remote registry access have been changed. Therefore, the logon attempt is not successful.”
Right now, this is a hotfix that you need to call in to get. If it becomes available for public download the link will be in the above article. This hotfix requires that you have Exchange Server 2003 SP1. Another article will be available to address the issue if you do not have Exchange Server 2003 SP1 installed.As always… please test the hotfixes in your test lab before deploying to production servers.