ciunkos.com/Glossary

Email authentication

Hard bounce

A permanent delivery failure: the address or its domain does not exist or refuses mail.

Receiving servers answer a message to a non-existent mailbox or domain with a permanent error (5xx). Mailbox providers watch the share of such bounces per sender; a high rate on a cold campaign is a common reason for a sending domain or IP to be throttled or blocked. Addresses that hard-bounce once should be suppressed for good, and lists should be checked before sending.

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Related terms

  • SPF (Sender Policy Framework): A DNS record listing which servers may send email for your domain.
  • DMARC: The policy that tells receivers what to do when mail fails SPF and DKIM, and where to report.
  • Null MX: A DNS record (MX 0 .) that says a domain accepts no email at all (RFC 7505).

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