Event stream
Per-message lifecycle events (delivered, bounced, opened, clicked).
Most providers expose lifecycle events for each message: accepted, delivered, bounced, complained, opened, clicked. Event streams power dashboards, downstream automation, and forensic debugging. Webhook delivery is the standard transport.
reading this as SaaS product teams
A SaaS product sends on behalf of its customers, so the deciding factors are tenant isolation, whether suppressions can be scoped rather than global, and whether support can answer why one specific account did not receive one specific email. Lifecycle automation and transactional sending usually need to coexist.
Applied to event stream, that means weighing multi-tenant support, event stream, the suppressions API, and operating track record ahead of the rest, against trial expiry notices, dunning, onboarding sequences, and per-tenant transactional mail.