Users can read and compose emails without network connectivity, such as when traveling. Second, Outlook cached mode allows users to work with mailbox data while offline. They use Outlook over slow networks such as home internet connections and public Wi-Fi. Users connect to their mailbox over longer distances. These days, Exchange mailboxes are hosted on servers on centralized datacenters, or in the cloud in Office 365. In the old days, when Outlook was primarily used to connect to Exchange servers that were close to the end user, on-line mode was good enough. Outlook performance in “on-line mode” is not all that great. When you connect Outlook to an Exchange account, Outlook downloads a copy of your mailbox content to a local cache. One of the biggest factors that influences Outlook performance is caching.
In other words, is Outlook running nicely, or is it running slowly? While all that is important, the end-users think of email as the client experience. As Exchange administrators we tend to think about servers, networks, and datacenters.
