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 teams choosing a transactional API
Judged purely as an API. What the request contract guarantees on retry, how complete the webhook event coverage is, whether suppressions are readable and writable programmatically, and how much the message log retains when something needs debugging in production.
Applied to suppressions, that means weighing idempotency keys, webhook coverage, event stream, and operating track record ahead of the rest, against high-frequency programmatic sends where a missed delivery is a product failure.