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 developers picking email infrastructure
The question here is which one a backend engineer would rather operate. That means SDK depth in the language already in use, whether a retry can safely be replayed, how much the event log tells you when a customer says the email never arrived, and whether the pricing curve stays sane as volume grows.
Applied to event stream, that means weighing developer experience, SDK breadth, idempotency keys, and operating track record ahead of the rest, against password resets, receipts, and magic links shipped from application code.