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 building autonomous agents
An agent stack treats email as a two-way primitive. Inbound has to arrive as structured JSON a tool call can consume, sends have to be safe to retry when a model loop repeats itself, and every action needs an audit trail. A provider without inbound parsing only solves half the problem.
Applied to event stream, that means weighing inbound parsing, idempotency keys, webhook coverage, and operating track record ahead of the rest, against inbound parsing into tool calls and model-generated sends that must not duplicate.