Activation email works when it reacts to what a user has already tried to do. The best programs start with a small set of product events and a clear definition of first value.
Map each message to one job: confirm context, remove friction, introduce a next action or invite a higher-touch conversation. If an email cannot be tied to a milestone, it is probably noise.
Review activation journeys weekly. Look at conversion, unsubscribes and reply themes together so the program improves with product behavior rather than vanity metrics.