Alerts › rules. Kinds sit under the tags. Downtime is one tick. None ticked still means every kind.
Open Alerts › rules and press + rule. Under kinds, tick downtime and leave the others clear. None ticked still means every kind. The address is typed and not added yet. Type and tags left empty cover every machine.
Press add contact. The grey sentence reads: tell email ops@example.com when a warning or worse (downtime) opens on 6 machines in Example Co, and again when it resolves. Press add rule when that sentence is the one you want. This shot stops before that. A later edit of the same rule is Change an alert rule.
A script that starts when a machine goes offline is Run a script only when a machine goes offline. A rule for every kind is Tell someone when a machine needs attention. Disk only is Only tell them about disk alerts. A new external IP or a WAN failover is Only tell them when the network path changes. A Docker app is Only tell them about a Docker app. A failing check is Only tell them about a failing check. A reboot is Only tell them about a reboot. A custom metric is Only tell them about a custom metric. A desktop that should stay quiet overnight is Leave a desktop out of offline alerts.