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 developers picking email infrastructure
The question here is which one a backend engineer would rather operate. That means SDK depth in the language already in use, whether a retry can safely be replayed, how much the event log tells you when a customer says the email never arrived, and whether the pricing curve stays sane as volume grows.
Applied to hard bounce, that means weighing developer experience, SDK breadth, idempotency keys, and operating track record ahead of the rest, against password resets, receipts, and magic links shipped from application code.