The Exchange Recipient Update Service

Rebuilds
Rather than apply individual policies or wait for the RUS to process updates, some administrators might consider forcing the RUS to rebuild the address lists for a domain. However, you should approach this action cautiously, especially if your organization uses an external program to generate email addresses and resynchronize them with AD. Because the RUS knows nothing about the processing that external programs perform, the RUS could end up allocating addresses that are under the control of the external program, leading to duplicate email addresses and message delivery failure.

When you tell the RUS to rebuild the address lists for a domain, you force it to reset the value of the USNchanged attribute to 1; this action then causes the RUS to process every mail-enabled object in the domain. Processing will start at the next scheduled interval unless you follow the Rebuild command with the Update Now command, in which case the processing will begin immediately. As you can imagine, executing the necessary LDAP queries and other required processing can take some time on domains that have tens of thousands of mail-enabled objects. In some deployments, a rebuild can take well over a day to finish, even when the Exchange server and domain controller (DC) involved in the operation are tightly coupled. Rebuild operations also generate a lot of work for the servers that are involved and keep the network busy accommodating the LDAP queries and AD replication traffic that results from the rebuild. All in all, I don't recommend performing a rebuild unless you are certain that doing so is the only way to fix a problem with an address list, such as a persistent failure to flush invalid or obsolete information from the GAL or to show new and updated information in the GAL. . . .