Deliverability & the inbox
Hard bounce
Also known as: permanent bounce
A delivery failure the mailbox provider signals as permanent — invalid address, domain doesn't exist, mailbox closed. The address must be immediately suppressed; future sends damage sender reputation.
A hard bounce is an SMTP 5xx error signalling permanent delivery failure. Common causes: address doesn't exist (user@nonexistent.example), domain doesn't resolve, mailbox is closed, recipient's server has blocked the sender entirely. Every hard bounce must be added to the suppression list within hours — continuing to send to a hard-bounced address is a strong spam signal to mailbox providers and can damage sender reputation across all future sends. Most ESPs auto-suppress on hard bounce; operators should verify this is happening (quarterly audit) and ensure the suppression applies across the entire account, not just the specific sending stream. Hard-bounce rate above 2% is the operator threshold for "something is wrong with list acquisition" — usually scraped lists, typo traps, or bad data from a failed integration.
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