Change how much mail to keep offline
With a Microsoft Exchange Server account, you decide how much mail is available when working offline with a slow or no network connection.
Click File > Account Settings > Account Settings.
Select an Exchange account, and then click Change.
Under Offline Settings, set the slider to All, 12 months, 6 months, 3 months, or 1 month. Cached Exchange Mode, which downloads a copy of your messages, must be turned on.