According to Microsoft Documentation there is no way to run Exchange 5.5 on Windows 2003 -in a general way-. But...in the document named "Microsoft Exchange Server Compatibility with Microsoft Windows Server Operating Systems" is written: "Although Exchange 5.5 and Exchange 2000 should not be installed on servers running Windows Server 2003, Windows 2000 file and print servers, domain controllers, and global catalog servers can all be upgraded to Windows Server 2003 with no impact on Exchange" The questions is : It is possible to upgrade and Exchange 5.5 SP3 running on Windows 2000 SP3 Domain Controller (mixed mode member server) to Windows 2003, and still be able use the Exchange Server 5.5 with no damage or conflit between the directories or lost connectivity? This is my case for upgrade, but I have not found any case like this, I hope somebody knows.! Greetings to all