Problem

What is a NDR (Non delivery report)?

Solution

A Non-Delivery Report (NDR) also known as a bounce message, Delivery Status Notification (DSN), or Non-Delivery Notification (NDN) is an automated email generated by a mail system to let the original sender know that a message failed to be delivered. In other words, the email “bounced.”

These messages usually include SMTP status codes (such as 4.x.x or 5.x.x) which indicate the type of delivery failure. The NDR is typically sent with an empty return-path (shown as "<>") so that further bounces are not triggered.

Note: Out-of-office (OOO) replies are sometimes treated as a special category of NDRs or DSNs, though they aren’t “failures” in the same sense.

Why Do Emails Bounce? (Common Causes)

Here are some frequent reasons why messages are returned with an NDR:

• Invalid recipient address: typo, non-existent domain or mailbox
• Mailbox full: the recipient’s inbox is over quota
• Domain or server issues: DNS records incorrect, domain expired, or mail server down
• Blocked or blacklisted: your sending IP or domain is blocked by spam filters
• Policy restrictions: recipient’s server refuses your connection (e.g. greylisting or rate-limits)
• Misconfigured mail system: missing SPF, DKIM, DMARC or other authentication failures
• Temporary problems: transient network or routing issues, denoted by 4.x.x codes

The numeric SMTP codes are clues:

• 4.x.x = temporary error (retryable)
• 5.x.x = permanent error (non-retryable)

By examining the code and message, you can usually deduce whether the issue might resolve over time or whether you need to take action on your side (e.g. correct address, remove bounce list, or talk with your mail provider).

How Does OutMail Handle NDRs?

By default, OutMail does not send outbound NDRs. The rationale is that many email servers incorrectly accept messages and later generate bounce responses, which can create back-scatter spam across the Internet.

Back-scatter occurs when a server sends NDRs in response to spam or forged sender addresses causing the NDR to go to an innocent third party. Because of that risk, suppressing outbound bounces can reduce unintended “bounce spam.”

If your use case requires outbound NDRs (or out-of-office replies) from your sending domain, you can contact Prolateral support to enable and configure NDR/OOO for your OutMail account. (See our related article for specifics.)

Diagnosing and Managing Bounces

If your emails are bouncing, here’s a quick checklist to help you troubleshoot:

The outMail logs (within the portal) can also help to analyse any delivery issues.

Best Practices to Avoid Bounces

• Maintain a clean mailing list, remove invalid or inactive addresses periodically
• Use double opt-in to confirm addresses
• Have good opt-out mechanisms in the body and headers
• Monitor bounce rate thresholds (e.g. keep the rate > 1–2%)
• Authenticate your domain properly (SPF, DKIM, DMARC)
• Warm up new IP addresses gradually
• Avoid spammy language or attachments
• Respect recipient server limits and standards

When Might You Want Outbound NDRs?

Some advanced use cases may benefit from enabling outbound NDR or OOO:

• Automated systems sending notifications where bounce feedback is needed
• Internal business systems wanting sender awareness of failed deliveries
• Delegated or managed email systems with compliance or auditing requirements

If you need this functionality, You can request to enable the NDR feature within the management portal under the outMail section.