Hard bounce
A permanent failure — the address does not exist or will never accept mail. Stop sending.
Say it out loud
“That address does not exist. We stop sending to it immediately and permanently.”
A hard bounce is the receiving server saying the failure is permanent: unknown user, dead domain, blocked outright. Mainstream platforms suppress these automatically. Continuing to mail them is one of the clearest signals of a list nobody is maintaining.
5xx = permanent. That is the entire rule: read the first digit.
4xx = temporary. The server is asking you to come back later, not telling you to stop.
First digit 5, stop forever. First digit 4, try again. Everything else in bounce handling is detail.
Your platform
Klaviyo, Mailchimp and friends do this automatically.
What goes wrong
Re-importing an old list that reintroduces addresses your platform already suppressed. Suppression lists exist for a reason and a CSV upload is how they get overruled.
The dated rule behind this
A definition is not a citation. These are the pages with the primary source, the date it was published, and what to do about it.
See also
Where this sits
Stop 6, the verdict lands. The message goes to an inbox, a spam folder, a refusal, or an address that was never a person.