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 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 hard bounce, 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.