NOTE: Cached Exchange Mode creates a self-synchronized copy of your messages that are stored on both the server and your client.
STEP 1: Click Tools -> Account Settings.
STEP 2: Select your email account and click Change.
STEP 3: You can enable/disable "Cached Exchange Mode" by doing the following:
Then click Next.
STEP 4: Click Next, then click Finish.
NOTE: You may have to close and re-open Outlook for the changes to take effect.