To enable Notifications, click on the small arrow next to your user name at the upper left of the web portal:
Next, click on "Account":
Scroll down until you find the "Notifications" switches. By default, Notifications are disabled.
Click on the slider controls on the right hand side to enable or disable notifications. You can choose to enable or disable notifications for devices which you own, as well as devices which were shared to you from a different account.
Online notifications will be sent almost immediately.
Offline notifications will be sent almost immediately, if the system was shutdown or rebooted using specific "shutdown" or "reboot" commands. Using command line options to stop and start the connectd daemons directly will also send notifications immediately.
If the reason for your device going offline is an unexpected interruption of power or internet connection, it may take up to 15 minutes to receive the offline notification.