If, under the heading of Email in the Settings of the Garmin Connect Mobile app for Android, you only see a toggle for Gmail, then you should be adding more notification-capable apps. I don't know which app you use to collect/receive messages from Microsoft Outlook Exchange server (huh?), but if that app is available for selection from the Select Application screen when you go to add more toggles, then just pick it; whether that app will then generate notifications for new messages is its business and not Garmin Connect Mobile's.
On Android GCM, under "Settings>Smart Notifications", you can turn on/off some of the basic apps (like gmail), but with the three dots in the upper right, you can also add other apps that can send notifications to the watch.
