Glossary · Email

Email Bounce (Hard and Soft)

An email bounce is a failed delivery notice from a mail server, with a 4xx or 5xx SMTP code showing whether the failure is temporary or permanent.

Updated October 5, 2026

An email bounce is a message returned, or a refusal during the SMTP conversation, saying a message was not delivered. The reply carries a three-digit SMTP code and often an enhanced status code such as 5.1.1.

Why it matters

Codes starting with 4 are temporary (soft) failures: the sending server retries for a period, typically several days, before giving up. Codes starting with 5 are permanent (hard) failures: retrying will not help. Reading the code and text tells you whether the cause is a bad address, a full mailbox, a missing MX record, failed authentication or a blocklisting. Repeatedly sending to addresses that hard bounce hurts your sender reputation.

Example

550 5.1.1 <jane@example.com>: Recipient address rejected: User unknown
550 5.7.26 This message does not pass authentication checks (SPF and DKIM both do not pass)
451 4.7.1 Greylisted, please try again later

The first is a hard bounce for a nonexistent mailbox, the second a policy rejection due to authentication, the third a soft failure that usually clears on retry.

Common mistakes

  • Treating every bounce as a bad address. A 5.7.x code is usually about policy or reputation.
  • Continuing to mail addresses that have hard bounced.
  • Ignoring the full text of the bounce, which often names the exact check that failed.
  • Confusing a bounce with spam-folder placement, where nothing is returned.

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