From time to time organizations have multiple Windows 
domains that host user's accounts but may only have one 
domain that hosts their Exchange  servers. In these 
scenarios we need to create additional Recipient Update 
Services, or our Global Address List, will not be populated 
with the account information from the domains that do not 
host the Exchange  server.

Recipient Updates in Multiple Domain Environments
=======================================================
Each domain where mail-enabled objects will exist requires 
a Recipient Update Ser-vice, and some domains may require 
several instances of the Recipient Update Service. This may 
require you to create additional instances of the Recipient 
Update Service. 
You can have multiple instances of the Recipient Update 
Service on an Exchange 2003 computer, but each domain 
controller in a domain can only participate in one 
Recipient Update Service. If you want multiple Recipient 
Update Service instances for a domain, that domain must 
have multiple domain controllers. 
The Exchange 2003 computer and the Windows domain 
controller that are associated with a particular Recipient 
Update Service are displayed in the details pane of 
Exchange System Manager. Click the Recipient Update 
Services container in the con-sole tree to display this 
information in table form. 
When you are creating a Recipient Update Service, you 
cannot select the domain controller; you can only select 
the domain that you want the new Recipient Update Service 
to be associated with. After you have finished creating the 
Recipient Update Service, you can edit the properties of 
the Recipient Update Service and select the domain 
controller that you want.