Feedback loop
A mailbox-provider feed of spam complaints.
A feedback loop sends complaint events back to a sender or ESP when recipients mark messages as spam. Not every mailbox provider offers one, and complaint data is often aggregated or anonymized. Providers use it to suppress complainers and protect sender reputation.
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 feedback loop, 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.