That all domains which have been "domain-prepped" have a RUS is an Exchange 2000/2003 requirement which is unchanged for the co-existence scenario.
Exchange Server 2003 communicates with directory servers to update recipient objects (such as mailbox-enabled user accounts and mail-enabled groups) with e-mail addresses, according to the recipient policies defined for the organization.
Domain Prep needs to be run in any domain that has recipients, Exchange servers, or global catalogs.
Additionally, a Recipient Update Service (RUS) needs to be created that points to a domain controller in that domain.
This is typically done in a hosting scenario where multiple customers with different SMTP/DNS domains are hosted in a single AD forest.
Recipient Update Service (RUS) in a multi-domain forest: If your forest has multiple domains where recipients reside, you will need to run Domain Prep in those domains.
Home: Introduction Issue #1: Exchange Server storage sizing and location Issue #2: SMTP virtual server and connector configuration Issue #3: Exchange recipient policies and Recipient Update Service Issue #4: Exchange Server messaging hygiene Issue #5: Exchange Server and DNS Issue #6: Front-end/back-end Exchange Server topology issues Issue #7: Exchange Server information stores and mailbox sizes Issue #8: Moving or removing Exchange servers Issue #9: Exchange Server backups and disaster recovery Issue #10: Exchange Server monitoring -- or lack thereof Bharat Suneja is a Microsoft Certified Trainer (MCT), Exchange MVP, and Principal Exchange Architect for Zenprise, Inc., maker of real-time troubleshooting and diagnostics software for Exchange.