Outlook 365 - Work Offline
Offline mode allows you to view cached emails and add them to your sent que while disconnected from the network.
Cached Exchange Mode must be enabled in order to use Microsoft Outlook in Offline Mode.
Change how much mail to keep offline
Click File > Account Settings > Account Settings.
Select an Exchange or Microsoft 365 account, and then click Change.
Under Offline Settings, move the slider to the desired amount of time, such as All, 12 months, 6 months, 3 months, or 1 month. Note: If you're using Outlook 2016, you can also select an amount of time as short as 3 days.
For example, if you move the slider to 3 months, only mail from the past three months will be synchronized to your Offline Outlook Data (.ost) file from the Exchange server. Items older than 3 months reside only in your mailbox on the server. You can change this setting at any time.