Global Messaging

Exchange Server 200x is a complex messaging system with several Exchange core components and services which work together to provide an efficient email system. Exchange Server 200x highly depends on Microsoft Active Directory and a correctly functioning DNS system.
Exchange 200x services access information that is stored in Active Directory and write information to Active Directory. If this communication occurred directly between each service and Active Directory, Exchange 200x could overwhelm an Active Directory domain controller with communication requests.

-Sudhir