Now that there are so many different versions of Windows/Active Directory you can easily get confused to what Exchange Server version is supported on which OS/Active Directory
Luckily there is a support matrix that explains this very clearly!

This is a screenshot of the Active Directory Support Matrix

On this page there is information for Support Matrix:

• Release Model
• Supported Operating System Platforms
• Supported Active Directory Environments
• Web Browsers That Are Supported for Use with the Premium Version of Outlook Web Access
• Web Browsers That Are Supported for Use with the Basic Version of Outlook Web Access µ
• Web Browsers That Are Supported for the Use of S/MIME with Outlook Web Access
• Clients
• The Microsoft .NET Framework
• Windows PowerShell
• Microsoft Management Console
• Windows Installer

Exchange Server Supportability Matrix:
http://technet.microsoft.com/en-us/library/ee338574.aspx