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.
Cached Exchange Mode, which downloads a copy of your messages, must be turned on.
By default, Outlook configures Exchange and Microsoft 365 accounts to only synchronize 12 months of your email.
Changing the amount of data to synchronize can affect the time it takes Outlook to open as well as the amount of storage space Outlook uses on your computer. If you're running in a virtual desktop environment, we recommend you only synchronize one month of data.