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 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 sender reputation, 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.