Outlook hangs updating cached messages
When you use Outlook to view the folder, items that are older than this expiration date appear to be expired as expected.
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.
Summary: Learn about the two Exchange connectivity modes that you can use for Outlook 2013 deployments—Cached Exchange Mode and Online Mode.
Audience: IT Professionals Decide which connectivity mode, Cached Exchange Mode or Online Mode, is appropriate for your environment.
If a user disconnects from the network—for example, by removing a portable computer, such as a laptop, from a docking station—the latest Outlook information is automatically available offline.
Online Mode works by using information directly from the server, and, as the name implies, it requires a connection.