When invited to a meeting or event by another user, you will have the event automatically added to your schedule and marked as tentative. This is done to avoid a third party attempting to schedule an event with you at the same time – if they view your free/busy information they will see that you may be occupied. However, that default behavior may not be suitable for certain users, and they may desire to turn it off. To turn off this feature:

1. Open the Outlook desktop client.
2. Click the File tab at the top of the program window.
3. Select Options from the list on the left. This will cause a popup to appear.
4. Select Mail from its left pane, then scroll down to Tracking.
5. Uncheck Automatically process meeting requests and responses to meeting requests and polls.