Glossary · Security and privacy
hard bounce
A permanent delivery failure: the mailbox does not exist. Sending systems respond by suppressing the address so they stop hammering it, which means every later message is accepted by the sending API and dropped, with no error, until the suppression is cleared.
What hard bounce means
A permanent delivery failure: the mailbox does not exist. Sending systems respond by suppressing the address so they stop hammering it, which means every later message is accepted by the sending API and dropped, with no error, until the suppression is cleared. It is the failure that looks like success from the sending side.
What hard bounce can and cannot support
| It can support | It cannot support |
|---|---|
| Sending systems respond by suppressing the address so they stop hammering it, which means every later message is accepted by the sending API and dropped, with no error, until the suppression is cleared. It is the failure that looks like success from the sending side. | A permanent delivery failure: the mailbox does not exist. |
Where hard bounce comes up on this site
- Five days of our alerts went to an address with no mailboxWhat a hard bounce does to a sending system The next message to hello@ reached Google's servers, which now held the domain's MX, and Google answered that the user did not exist.
Related terms in Security and privacy
Where an agent, a tool or a page can be turned against its operator, and the controls that limit the damage without proving safety.
Indirect prompt injection · Direct prompt injection · Jailbreak · Data exfiltration · Sensitive information disclosure · Secret leakage · Insecure output handling · Excessive agency · Least privilege · Trust boundary · Input validation · Output validation · Allowlist · Denylist · SSRF · RCE · Credential scope · OAuth scope · API key · Bearer token · mTLS · Rate-limit policy · Audit log · PII · Data minimization · Purpose limitation · Retention period · Data residency · Tenant isolation · Threat model · DMARC · DMARC alignment · SPF · DKIM · MTA-STS · TLS-RPT · CAA record · DNSSEC
Questions about hard bounce
What is hard bounce?
A permanent delivery failure: the mailbox does not exist. Sending systems respond by suppressing the address so they stop hammering it, which means every later message is accepted by the sending API and dropped, with no error, until the suppression is cleared.
What does hard bounce not show or guarantee?
A permanent delivery failure: the mailbox does not exist.
Which area of the glossary does hard bounce belong to?
Security and privacy: Where an agent, a tool or a page can be turned against its operator, and the controls that limit the damage without proving safety.
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