Complaint rate
The share of recipients who mark email as spam.
Complaint rate is one of the strongest negative sender reputation signals. It is usually measured as spam complaints divided by delivered messages. High complaint rates often mean poor consent, misleading subject lines, stale lists, or irrelevant targeting.
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 complaint rate, 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.