Hard bounce
A permanent delivery failure — the address doesn't exist, the domain is invalid, or the recipient blocked the sender.
A hard bounce is a permanent email delivery failure. The receiving server has confirmed that the address won't accept mail now or in the future — the mailbox doesn't exist, the domain is invalid, the recipient explicitly blocked the sender, or the recipient's server is permanently unreachable.
Hard bounces should be removed from your sending list immediately. Re-sending to a hard-bounced address generates additional bounces, which directly damages sender reputation. Most ESPs automatically suppress hard-bounced addresses after the first bounce; some require manual list hygiene.
Hard-bounce rates are a leading indicator of list-acquisition problems. Healthy lists run under 1% hard bounces. Above 2% triggers ESP warnings; above 5% leads to account suspension on most platforms. The structural fix is double opt-in plus regular list validation (services like ZeroBounce or NeverBounce verify addresses before send).