A hard bounce is a permanent delivery failure. The receiving server refuses the email because the address doesn't exist, the domain doesn't exist, or the domain has no mail server. Resending won't help.
What it looks like
The bounce message carries a 5xx code, most often 550 5.1.1 ("user unknown" or "address doesn't exist").
Why it hurts
A high share of hard bounces tells providers you're emailing addresses you didn't get from the people themselves. Keep the overall bounce rate under 2%; above that, a new domain's reputation drops fast.
What to do
- Remove a hard-bounced address after the first bounce, for good, from every list and campaign.
- Verify lists right before sending: most hard bounces come from addresses that died since the list was built.
- Stop a campaign whose bounces climb, and find the bad source.
Compare with a soft bounce, which is temporary. Check addresses with the free email validator.