Soft bounce
A temporary delivery failure.
A soft bounce means delivery failed for a temporary reason such as a full mailbox, greylisting, rate limiting, or a transient server issue. Providers usually retry soft bounces before converting them into final failures.
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 soft 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.