To enable Edge Lighting notifications, open your device settings > tap Display > Edge Screen, and then, enable the toggle for Edge Lighting. This will turn on the Edge Lighting feature for your device.
Once you've done this, you'll need to specify a higher priority notification for Outlook. To do this, tap Manage notifications > Select Outlook > Scroll to notifications > Sound > Notification Style (or Importance) > Select the option for Sound and pop-up (or Urgent).