Microsoft 365 - How To
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      Change How Much Mail to Keep Offline in Outlook

      By default, Outlook configures Exchange and Office 365 accounts to only synchronize 12 months of your email.

      1. Click File > Account Settings > Account Settings.

        Account Settings in the Backstage view

      2. Select an Exchange or Office 365 account, and then click Change.

        Change command in the Account Settings dialog box

      3. 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.

        Offline mail storage amount slider

        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.

      Note:

      • Cached Exchange Mode, which downloads a copy of your messages, must be turned on.
      • If you're using Outlook with an Office 365 account, we recommend you use Cached Exchange Mode.

       

      Source: https://support.office.com/en-us/article/work-offline-in-outlook-f3a1251c-6dd5-4208-aef9-7c8c9522d633