A hard bounce corresponds to a permanent SMTP failure, most commonly a 5xx response like 550 5.1.1 (mailbox does not exist). It means the recipient address is invalid, not that the message failed for a transient reason.
Retrying a hard-bounced address is one of the fastest ways to damage sender reputation: it signals to mailbox providers that a sender doesn't maintain list hygiene. Hard bounces should suppress the address immediately, not after repeated failures.
A hard-bounce rate consistently above roughly 2% of a send is a strong signal of stale or unverified list data, independent of any single address's status.