Microsoft just released a new paper on this. When running Microsoft® Exchange Server 2003 or Exchange 2000 Server in larger environments, the frequency of queries to the Active Directory® directory service can be very high. Exchange Server uses its directory access component to communicate with Active Directory domain controllers and global catalog servers to perform tasks such as e-mail address lookups, distribution group expansion, Microsoft Outlook® client proxy, and referral services. With such a heavy load being placed on domain controllers, Microsoft IT optimized the performance of Exchange when communicating with Active Directory by creating a new Active Directory site and isolating domain controllers and global catalog servers just for Exchange. Get it here.