Outlook 2013 caches a local copy the user’s mailbox in the background to prepare the user for offline use without affecting the user's experience.
The sync slider lets Outlook 2013 users limit the email messages that are synchronized locally in their Outlook data file (.ost).
By default, if Cached Exchange Mode is enabled, Outlook 2013 will cache email messages only from the last 12 months and remove anything older than 12 months from the local cache.
Virtualized or Remote Desktop Services (RDS) (Terminal Services) environments that run Outlook 2013 and on which disk size or disk input/output (I/O) limitations prevent running Cached Exchange Mode at the scale you want.
If users work with a large mailbox, you can reduce the size of the local data file by using Cached Exchange Mode with the sync slider enabled.
The email messages that are removed from the local cache are still available for users to view, but they’ll need to be connected to the Exchange Server computer to view them.
Users can scroll to the end of the email list in a folder and then choose the message Click here to view more on Microsoft Exchange to view the email messages that were removed from the local cache.
This is just one way to improve the performance of Cached Exchange Mode for users.
Outlook 2013 supports running in Cached Exchange Mode in a Remote Desktop Services (Terminal Services) environment that has multiple users.
At setup, the user can decide to enable Cached Exchange Mode, or the user can control this setting by using the Use Cached Exchange Mode for new and existing Outlook profiles option in the Office Customization Tool or in Group Policy.
Even when it is configured in Cached Exchange Mode, Outlook 2013 must contact the server directly to do certain operations.
In these environments, we also recommend that you consider using Encrypting File System (EFS) or Bit Locker with Cached Exchange Mode as a robust solution.