Cached Exchange Mode enables a better experience when you use an Exchange account. In this mode, a copy of your mailbox is saved on your computer. This copy provides quick access to your data, and it is frequently updated with the server that runs Microsoft Exchange. When using Outlook with an Office 365 mailbox, we recommend always using Cached Exchange Mode.
To start open your GPO and go to:
User Configuration -> expand Administrative Templates -> Microsoft Outlook 2016 or 2013 -> Account Settings -> Exchange Double -> Cached Exchange Mode.
In the Settings Windows Select Enable and Number of months