My fellow MVP, Alexandre Janeiro, was reading this new Exchange KB article, You cannot add users to a private address book in Outlook when the forest functional level is set to Windows Server 2003, when he spotted an error. The article states the following:
Consider the following scenario:
• You install Microsoft Exchange 2000 Server or Microsoft Exchange Server 2003 on a Microsoft Windows Server 2003-based computer.
The last time I checked, installing Exchange 2000 Server on Windows Server 2003 was not an option. But just to be sure, I made some investigation and found the proof:
Meanwhile, Alexandre has already sent feedback to Microsoft.