Hard Bounce
A permanent email delivery failure (SMTP 5xx), typically because the address does not exist or is blocked.
A hard bounce is a permanent SMTP rejection: the recipient mailbox does not exist (550 5.1.1), the domain has no MX (550 5.1.2), the sender is blocked, or policy refuses the message. Hard-bounced addresses must be suppressed immediately, because retrying them piles up more bounces, which mailbox providers interpret as a list-hygiene signal and translate into reputation damage. Most ESPs auto-suppress on the first hard bounce. Compare with soft bounce, which is a temporary 4xx failure that should be retried.