Microsoft has a KB article on this here.
To resolve this problem, add the Exchange Domain Servers group from the child domain to the Exchange Enterprise Servers group in the parent domain.
You can also resolve this problem by creating the Recipient Update Service (RUS) in the root domain.
For additional information about how to create an RUS for Exchange 2000 Server and Exchange 2003 Server, view the article in the Microsoft Knowledge Base:
275294: Recipient Update Service Is Not Created Automatically