This seems like kind of a dumb question. The last time I dealt with mixed mode Active Directory was with Server 2000 and 2003. After that I have always just upgrade 2003 domains to 2008 R2 and installed new domain controllers. So this client is still using Windows Server 2003 Standard (not R2) with Exchange 2010. They have two of these Server 2003 domain controllers in their office and Exchange 2010 resides in one of our datacenters with a site to site back to their primary office on the same subnet without a domain controller in our datacenter. That will change in the future. I am now setting up a second site in a different datacenter so they have DAG for high availability. This is it's own subnet and will have it's own domain controller. I would like to use Windows Server 2008 R2, but before I run forestprep and domainprep, I want to make sure I am not going to cause an issue with their Server 2003 servers, especially winlogon. I can only imagine them coming in on Monday and not being able to authenticate on the network.