I wanted to post a follow-up to Cathy's documentation announcement with a bit more information on RPC Client Throttling.
With the release of Microsoft Exchange Server 2007, a new feature known as RPC Client Throttling is available to administrators to help manage the end-user performance experience. RPC Client Throttling was introduced to help prevent client applications from sending too many RPC operations per second to the Exchange server which could decrease overall server performance. These client applications include desktop search engines searching through every object inside a user's mailbox, custom applications written to manipulate data that is located in Exchange mailboxes, enterprise-class e-mail archiving products, or CRM enabled mailboxes with e-mail auto-tagging enabled. Client throttling allows Exchange to identify and help prevent server monopolization by a few users. When a client is identified by the Exchange server as causing a disproportionate effect on the server, the server will send a "back-off" request to the client to reduce the performance effect on the server. To learn more about RPC Client Throttling, see Understanding Exchange 2007 Client Throttling.