Non-Delivery Report (NDR)
Updated
A non-delivery report (NDR), often called a bounce message, reports that an email could not be delivered. It may identify an invalid recipient, policy rejection, routing problem, or another failure. The useful evidence is the diagnostic information and original transaction context, not merely the subject line saying delivery failed.
What do I need to know about Non-Delivery Report (NDR)?
- Temporary delivery problems and permanent rejections need different handling; repeated blind resending can make matters worse.
- An unexpected bounce may be backscatter from forged sender information rather than proof that your account sent the original mail.
- Fake delivery-failure messages can themselves be phishing lures.
- Diagnostic details can contain addresses and internal information; share them through appropriate support channels.
An illustrative example
A finance application reports failed invoices after a domain-authentication change. The administrator compares the NDR’s reason with a test message’s authentication results and the application’s sending setup. Re-entering the recipient address repeatedly would not fix the underlying alignment problem.
Practical checks
- Confirm whether your organization actually sent the original message and locate its trace record.
- Review the status code, reporting server, recipient, time, and stated reason using the provider’s documentation.
- Correct the specific issue and test delivery without weakening unrelated security controls.
Five FAQs about Non-Delivery Report (NDR)
Does a bounce mean the recipient blocked me personally?
Not necessarily. Invalid addresses, policy decisions, routing failures, and authentication problems can all cause rejection. The diagnostic details provide the next investigation step.
Should I resend immediately?
First distinguish a temporary condition from a permanent failure. Your mail service may already retry temporary failures; permanent errors generally need correction.
Why did I receive a bounce for mail I never sent?
Your address may have been forged, creating backscatter. Check sent-mail and audit evidence before concluding that your mailbox was compromised.
Can a delivery-failure notice be phishing?
Yes. A notice that asks you to sign in through an unfamiliar link or download a viewer should be treated cautiously. Check delivery status through the known mail service.
What should I send to IT support?
Provide the original NDR, approximate send time, intended recipient, and the application used. Avoid exposing sensitive message content beyond the approved support process.
Sources and further reading
Microsoft: Non-delivery reports and SMTP errors · Microsoft: Mail flow in cloud organizations
