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 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 complaint rate, 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.