Using a Single Forest Topology
If your organization has a single Active Directory forest, you can implement Exchange in that forest. The single forest Exchange design is recommended because it offers the richest set of mail system features and has the most streamlined administrative model. Because all resources are contained in a single forest, a single global address list (GAL) contains all users across the entire forest. The following figure illustrates this scenario.
Two examples of implementing Exchange in a single Active Directory forest
The single forest option offers the following advantages:
Provides the richest set of mail system features
Allows for a streamlined administrative model
Leverages an existing Active Directory structure
Uses existing domain controllers and global catalog servers
Does not require provisioning or synchronization
The main disadvantage associated with this option is that administrators need to determine how to share or divide responsibilities for managing Active Directory and Exchange objects.