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 teams choosing a transactional API
Judged purely as an API. What the request contract guarantees on retry, how complete the webhook event coverage is, whether suppressions are readable and writable programmatically, and how much the message log retains when something needs debugging in production.
Applied to event stream, that means weighing idempotency keys, webhook coverage, event stream, and operating track record ahead of the rest, against high-frequency programmatic sends where a missed delivery is a product failure.