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 early-stage startups
At an early stage the deciding factors are what the free tier actually covers, what the first paid invoice looks like, and how much of a founding engineer's week the setup costs. Feature depth that only matters at a million sends a month is noise until it is not.
Applied to feedback loop, that means weighing free tier, best published rate per 1,000, entry price, and developer experience ahead of the rest, against a waitlist, a launch announcement, onboarding email, and the first transactional sends.