How to read an email bounce message and find the real cause
Bounce messages look noisy because they combine a friendly explanation, structured delivery fields, server replies, provider-specific policy text, and sometimes the original message. You only need a few lines to make the correct first decision.
Updated July 15, 2026
1. Find the affected recipient and action
Look for Final-Recipient and Action. In a message sent to several people, diagnose each recipient block separately; one failed address does not mean all recipients failed.
2. Decide permanent or temporary
Read Status or the SMTP reply. A 4-class result is temporary and normally belongs in a queue with backoff. A 5-class result is permanent and should not be sent again unchanged. A 2-class result indicates success at that stage.
3. Decode the enhanced status
Use the final two numbers to choose the problem family: 1.1 recipient, 2.2 mailbox quota, 3.4 message size, 4.1 destination host, or 7.1 policy. See the SMTP status code reference for the complete reading method.
4. Read the provider sentence and link
The prose after Diagnostic-Code often supplies the actionable cause: a misspelled address, a named blocklist, failed SPF, a complaint-rate threshold, or a provider policy identifier. Prefer an official provider postmaster link over third-party guesses.
5. Choose one cause-specific action
- Recipient: verify, correct, or suppress the address.
- Authentication: check the actual sender's SPF, DKIM signature, and DMARC alignment.
- Rate/reputation: reduce bursts, review complaints and list quality, then follow the receiver's remediation process.
- Routing: check destination MX and mail-server availability.
- Content/size: remove the named problem and resend only after the message changes.
For a private first pass, paste the complete notice into the email bounce analyzer. It runs locally and produces a sanitized summary you can share without copying recipient or message content.