Outlook always updating cached messages
With Cached Exchange Mode, you do not have to set up Send/Receive groups, choose folders that you want to be available offline, and then keep those folders synchronized.
About: Outlook 2016 for Mac stores messages and other items, such as calendar events, contacts, tasks, and notes, as a SQLite database on your computer.
You can also specify that updates to the Address Book be downloaded during synchronization.
If you use an Exchange Server email account (like all UW-Madison Office 365 users), we recommend that you use Cached Exchange Mode.
By default, Cached Exchange Mode creates and uses an Offline Folder file (.ost) and then downloads and maintains a synchronized copy of the items in all folders in your mailbox.
You work with the information on your computer, and Outlook synchronizes the information with the server.