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In addition to performing synchronous user actions, a server performs asynchronous, background actions: accepting, transferring, and delivering messages, making routing decisions, expanding distribution lists, replicating changes to public folders and directory service information, executing rules, and monitoring storage quotas. A server can perform these tasks asynchronously on behalf of users, whether they are connected or not. This work is referred to as asynchronous because the time it takes for these actions to be completed doesn't influence the users' perception of the system's speed, provided that the actions are completed within a reasonable amount of time.
In general, as with user actions, the load caused by background actions is proportional to the number of users on the server and the actions they perform. However, other factors, such as whether the server has connectors to other sites or systems, can have a significant impact. For dedicated connector or backbone servers, which don't directly support users, user actions place almost no load on the server. The load on the server is almost entirely the result of background actions.