Tell users something happened
In-app, email, or push — and the rules that stop it being spam.
Users need to know when <events>. Stack: <stack>. Design it: - Which channel for which event, and why. Don't email things that only matter while someone is looking at the page. - Per-user preferences, per event type, honoured everywhere. - Batching: five events in a minute should not be five emails. - A record of what was sent, so I can answer "did they get it?" - Sent in the background. A slow email provider must never hang a request or fail the action that triggered it. Then: what's the unsubscribe path, and is it legally sufficient for each channel?
Default to fewer notifications than you think. The cost of one too many is someone muting you forever; the cost of one too few is they check the page.