Suppressions
A list of recipients the provider refuses to send to.
Suppression lists hold addresses that bounced, complained, or unsubscribed. Most providers manage suppressions automatically and refuse to send to suppressed addresses. Some let you import or export the list; treating suppressions as authoritative protects sender reputation.
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 suppressions, 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.