A hard bounce is a permanent, non-recoverable email delivery failure. When you send a message to an address that does not exist, sits on a domain with no mail servers, or has been permanently disabled, the receiving system returns a 5xx SMTP error and the message is rejected for good. Retrying will not help.
Hard bounces are the most damaging kind of delivery failure for a sender's reputation. Mailbox providers interpret a high hard bounce rate as a signal that you are not maintaining your list - the hallmark of a spammer working from scraped or purchased data. Even a few percent can trigger throttling or blocklisting.
The most common causes are typos in the local part, addresses at domains that have expired or never had mail service, and employees who have left a company. Because these problems are permanent, the correct response is to immediately suppress the address so you never send to it again.
The best defense is prevention. Verifying addresses before you send - checking syntax, confirming the domain has MX records, and screening for known-dead patterns - removes most hard bounces before they ever occur. Cleaning your list this way protects deliverability far more effectively than reacting after bounces pile up.