How many Recipient Update Services (RUSs) should I have, and where should they be located?
By default, a newly created Exchange 2000 Server organization has two seemingly identical RUS entries: one for the whole organization and one for the domain. This duplication is necessary because the organization RUS updates entries in the forest's configuration naming context (NC), whereas the RUS in each domain updates objects that reside in the domain NC. Microsoft recommends that you have one RUS per domain. However, for domains that span large geographical or network areas, network latency might require you to add more RUSs.
Consider a single domain that includes servers in New York and Taipei, Taiwan. If you create a mailbox in New York, it won't be usable in Taipei until the attributes have completely replicated, but an RUS running in Taipei can make the necessary attribute adjustments almost immediately. For this configuration to work, though, you should put one RUS in each Active Directory (AD) site within a domain; that way, the RUS can contact a Global Catalog (GC) server that's near it on the network.