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 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 sender reputation, 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.