If you want a smooth deployment of Microsoft Exchange Server 2007, you'll need to make sure your Active Directory (AD) is properly designed and implemented. Here are some things you'll want to check.
Domain Controllers The schema master and all the Global Catalog (GC) servers within the forest where Exchange 2007 will be installed must be running Windows Server 2003 SP1 or later. All domains within the forest must have a functional level of Windows 2000 native or higher. Any site that will contain an Exchange server running the Mailbox, Hub Transport, or Client Access server role must contain at least one GC server. If possible, your domain controllers (DCs) should run a 64-bit Windows OS.
Global Catalog Servers You need to have one GC server core for every eight Exchange 2007 cores; your GC servers must be running a 64-bit Windows OS, and each server must have enough physical memory installed so that it can cache the entire AD database in RAM.
AD Site Topology Exchange 2007 routing is based on AD site topology. Placing Exchange 2007 in a dedicated site could negatively affect mail flow, particularly in organizations with five or more AD sites.
DNS Requirements The Mailbox, Client Access, Hub Transport, and Unified Messaging server roles must be domain members and must have their IP addresses registered with the organization's internal DNS server. The Client Access server needs to be accessible from outside the organization; for security, configure the firewall to use port forwarding to send HTTP traffic to the Client Access server. The Edge Transport server runs a hardened Exchange implementation and isn't a member of a domain.