Exchange includes a number of mailbox assistants to perform automated processing on different aspects of the data held in mailboxes. An assistant is just another name for a thread that executes within the Microsoft Exchange System Attendant process on a mailbox server. Exchange 2010 uses two types of assistants:
Event-based assistants: These respond to events as they occur. For example, the Resource Booking Assistant monitors requests to book rooms and lets users know if a room is unavailable.
Throttle-based assistants: These operate all the time to process data and are subject to throttling to ensure that their workload does not impact the responsiveness of a server. The Managed Folder Assistant is the most well-known of these ...