What is a hard bounce?
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A hard bounce is a permanent delivery failure. The receiving server says no with an SMTP 5xx code, and it won't work next time either. The usual causes are an address that doesn't exist (the infamous "user unknown" error), a domain that has expired, or a mailbox that was deleted or deactivated.
There's no point retrying. Suppress a hard-bounced address right after the first bounce. Most ESPs do it for you, and if yours doesn't, do it manually. Hard bounces are the ones that damage your sender reputation if you keep piling them up, and if your hard bounce rate jumps suddenly, that's an emergency, not a slow-burn issue.
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