What is Bounce Message?
Also known as: NDR
A bounce message, or non-delivery report (NDR), is an automated email sent to the originator when delivery cannot be completed, including a status code and failure reason.
A bounce message is an automatically generated email notification sent by a mail transfer agent (MTA) or mail delivery agent to the message's envelope sender when the original email cannot be delivered. It is formally defined as a Delivery Status Notification (DSN) in RFC 3464, though many systems still send simpler, non-standard bounce messages. The purpose is to inform the sender that delivery failed, along with the reason and often a numeric status code from RFC 3463.
When an MTA cannot deliver an email, it generates a bounce message addressed to the Return-Path (or envelope-from) address. The bounce describes the failure, such as a nonexistent recipient (550 5.1.1 user unknown), a full mailbox (452 4.2.2 mailbox full), or a content rejection (550 5.7.1 message blocked). The structure of a proper DSN includes a machine-readable body part (message/delivery-status) with per-recipient fields like status, remote-mta, diagnostic-code, and action. Modern MTAs, such as Postfix or Sendmail, produce DSN-compliant bounces unless configured otherwise.
In the email ecosystem, bounce messages play a critical role in backscatter spam and sender reputation. If a sender spoofs the envelope-from address, the genuine mailbox owner receives the unwanted bounce. This led to recommendations like SPF and DKIM to authenticate the sender, and to best practices where bounces should only be sent for locally authenticated or validated traffic. Bounce messages are distinguished from out-of-office replies (vacation notifications) which are non-delivery reports generated by the recipient's MDA, not the MTA. ISPs and email administrators monitor the bounce message rate to detect misconfigured servers or spam campaigns.
Key facts
- Defined in RFC 3464 as Delivery Status Notifications (DSN).
- Contains an RFC 3463 status code (e.g., 5.1.1 for permanent user unknown).
- Sent to the envelope Return-Path address, not the From header.
- Can cause backscatter if the envelope sender is spoofed.
- Should be distinguished from out-of-office auto-replies.
How it works in practice
Related terms
References
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