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 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 feedback loop, 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.