We recently updated this patch Microsoft Critical MS09-03/KB959897 for our Exchange 2003 servers . After that, just "one" of our Exchange Back-end server user objects are now getting HTTP Error 500 - Internal Server error. Note! Only one of our Back-end server users are experiencing this. The rest of exchange servers work just fine. I found kb841561 going back to 06 that details this error. The problem with that is, not all our exchange servers is impacted. I checked the user objects in AD and in the Exchange feature, Outlook web access is set to enabled. For a quick fix, I relocated the mailbox to another server and it works just fine. However, mailbox's on that entire server with the issue with all the stores in all Storage groups having this issue. Help!!!!