Hard bounce
A permanent delivery failure.
A hard bounce means the recipient address or domain cannot receive the message permanently. Common causes include nonexistent addresses, invalid domains, or recipient servers rejecting the address. Hard-bounced addresses should be suppressed quickly.
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 hard bounce, 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.