This should only be used when ram usage is unexplainably high. In my case it was using 8gb and causing the server to hit 95% or more ram usage at all times. By default, exchange will eat up ram to make itself run smoother and give up ram as needed to other programs but it doesn’t always work that way.
This will require restart of the exchange info store service when done so it should be done after hours.
Configuration -> Services -> Microsoft Exchange -> "Organization Name" -> Administrative Groups -> "Exchange Administrative Group" -> Servers -> "Server Name" -> InformationStore
Exchange 2010:
Exchange 2007:
When done, restart the exchange info store service and the ram usage should drop significantly.