Outlook can run your Exchange mailbox in 2 main modes; Online mode and Cache Mode. Both modes have their advantages and disadvantages.
Online Mode – Outlook gathers emails directly from the mail server and does not keep them locally.
Cache Mode – Outlook downloads a full copy of your entire mailbox locally and keeps it in sync with the mail server.
To change between the 2 modes, follow these steps.
Step 1 – Select the FILE menu from the top left of Outlook.
Step 2 – Click “Account Settings” box, then click “Account Settings” again.
Step 4 – You will see the “Use cache mode” tickbox as below. To use cache mode, tick the box and choose the amount of emails to cache. We recommend sliding it to include all emails if possible. To use Online mode, simply un-tick this box.
Step 5 – Click finish to complete the new setting. You will get a notification to restart Outlook, so simply restart your Outlook to complete.