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 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 complaint rate, 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.