What does an email bounce message mean?
A bounce is a delivery-status notice reporting a problem with an email. It may describe an incorrect address, a full mailbox, a message-size limit, an authentication refusal or another policy decision. The notice is more useful than the fact that an email is missing because it gives you a response to investigate.
This free bounce decoder recognises SMTP replies and enhanced delivery-status codes in pasted text. It explains supported codes and gives a cautious fallback for unfamiliar ones. It does not contact the recipient, send a test email or check your domain’s live DNS records.
How to use the bounce decoder
- Open the notice that came back after the failed message.
- Copy the diagnostic section, including the code and the words that follow it. Avoid pasting the original invoice, customer message or attachment.
- Choose Explain this bounce. Read each code alongside the original response if the notice includes several recipients or attempts.
- Follow the suggested checks and involve the owner of the affected service. Keep the unedited original privately for that investigation.
Common SMTP errors, without the guesswork
550 5.1.1 commonly points to a recipient address that was not recognised. Confirm the address before changing technical settings. 5.3.4 reports a message-size problem. 5.7.1 is a broad policy or permission refusal, so the wording after the code matters.
5.2.2 is particularly easy to misread: one provider may mean a full recipient mailbox while another describes a submission quota. Gmail’s 550 5.7.26 can describe different authentication refusals. A useful decoder should preserve that uncertainty rather than tell every business to change the same DNS record.
A temporary-class code beginning with 4 can appear in a final failure notice after retries have expired. A code beginning with 5 normally means that resending the unchanged message is unlikely to solve the reported condition. Read the notice’s action and full response before deciding to resend.
Keep the business problem in view
Identify the system that sent the failed message. A staff mailbox, a website form and an invoicing platform may use the same visible domain but different sending configurations. One successful mailbox test does not establish that a booking notification is correctly configured.
If a time-sensitive invoice or booking needs attention, confirm it through an established alternative contact method while the email problem is investigated. Do not send repeated duplicates or move sensitive documents to a public sharing link.
Questions about decoding bounce messages
Will this fix my email settings?
No. It explains the pasted response and suggests checks. DNS, account permissions and mail-routing changes belong with an authorised administrator who understands the affected service and can restore the previous settings.
What if it finds no error code?
Paste the diagnostic section rather than the customer message. If there is no bounce or rejection event, use the missing-email troubleshooter instead. A missing message alone does not identify the cause.
Is my bounce notice uploaded?
No. Decoding happens in this browser tab. The tool has no upload, account or AI service and does not save your paste. Use Clear when finished, especially on a shared device. Browser extensions and managed-device software remain outside this tool’s control.
Does a successful code mean the email reached the inbox?
No. A success-class response or provider acceptance event does not prove inbox placement, reading or the absence of later filtering.
Sources and further reading
Protocol references explain code and header meanings. Provider documentation supplies provider-specific context; the full message and receiving system still matter.
- Gmail SMTP errors and codes
- Microsoft email non-delivery reports
- Enhanced mail system status codes (RFC 3463)
Found a result that does not fit the original notice? Tell SenderCompass about the issue, without including account secrets or private customer content.