Sender reputation
The trust score mailbox providers infer from sending behavior.
Sender reputation is shaped by authentication, complaint rate, bounce rate, engagement, spam trap hits, sending consistency, and infrastructure history. It exists at multiple levels: domain, IP, envelope sender, and sometimes individual mailbox.
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 sender reputation, 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.