Exchange does not automatically select a new Exchange server to host RUS for a domain if the current server becomes unavailable.
Remember that RUS runs on an Exchange server, so it starts by querying a domain controller for any objects that have been updated since the last time it ran.
Let's say you recently added a mail-enabled user called New User1 with a logon name of newuser1.
Drill down under Recipients to find the Recipient Update Services folder.
In the right pane, you'll find at least two objects, one for Enterprise Configuration and one for each domain with an Exchange server. Figure 5.36 Recipient Update Service folder in ESM showing two RUS instances, one for Enterprise and one for the domain.
Exchange system objects reside in the Configuration naming context.