Outlook always updating cached messages
Exchange mail accounts by default use cached exchange mode, which makes Outlook store your recent messages and contacts on your computer for easier access.
Outlook periodically updates the local file, adding new messages and archiving older ones.
To do this, configure the Download shared non-mail folders option in the Office Customization Tool (OCT) when you customize your Cached Exchange Mode deployment.
Note that this setting applies to both mail and nonmail folders in Outlook 2016.
At setup, the user can decide to enable Cached Exchange Mode.
In some cases, you can improve the performance of Cached Exchange Mode for your whole organization or for a group of users—for example, users who work remotely.
Cached Exchange Mode is the preferred configuration in Outlook 2016 and is useful in the following situations: Even when it is configured in Cached Exchange Mode, Outlook 2016 must contact the server directly to do certain operations.If you must keep Outlook 2016 from creating a new compressed Outlook data file (.ost), use the Group Policy template for Outlook (Outlk16.admx) to enable the Do not create new OST file on upgrade policy setting.For more information about how to configure this and other settings, see Using Group Policy and the Office Customization Tool (OCT).Most users find that Cached Exchange Mode performs faster than online mode.However, several factors can influence a user's perception of Cached Exchange Mode performance, like hard disk size and speed, CPU speed, file size, and the expected level of performance.