Mail 550 first triage response

When this happens

Use this when only the visible message is known. Do not stop at the screen message; validate the first observable mismatch before any setting is changed first.

Symptom checklist

• 550 appears repeatedly in the Mail UI or logs.
• SPF, DKIM, DMARC, MX, raw bounce differs between successful and failed requests.
• The issue appears only after separating sender auth failure from receiver blocking.
• It often follows deploys, permission changes, configuration edits, or data refreshes.

Likely causes

• 550 specifically changes the investigation surface for Mail: verify the exact failing object, route, user, and timestamp before applying the broader pattern.
• The sending IP is not authorized by SPF or the envelope domain differs from the visible From domain.
• DKIM signs with a selector that DNS does not publish or receivers cannot verify.
• DMARC alignment fails even when SPF or DKIM individually pass.
• MX or SMTP auth points to an old provider after migration.
• A blacklist or recipient policy rejects the message after SMTP handoff.
• For the first triage case, the first useful clue is the first observable mismatch before any setting is changed.

First 1-minute checks

1. Write down the first failure time, latest change, affected user, path, and object ID.
2. Compare SPF, DKIM, DMARC, MX, raw bounce for success and failure in the same window.
3. Test the hypothesis: mail delivery failure caused by SPF/DKIM/DMARC alignment, MX routing, SMTP authentication, bounce policy, or blacklist reputation.
4. Classify this as first triage: the first observable mismatch before any setting is changed.
5. Capture current values before changing configuration.

First evidence

Treat 550 as a first triage case. First collect evidence for the first observable mismatch before any setting is changed.

Output examples

Normal output

No matching error appears during the affected request window.

Failing output

The same error appears at the exact request time with the affected path, user, or object.

Output-to-action branches

• Only the visible message is known.
  Collect timestamp, route, request ID, user, and the first backend log line before editing configuration.
• The working and failing outputs differ.
  Act on the differing layer first: For 550, apply the fix only after reproducing the same condition and saving the before/after evidence for this exact code.
• Command output is normal but users still fail.
  Separate browser cache, cookies, permissions, and network location before declaring it fixed.

Do not do this

• Do not change DNS, SSL, cache, and permissions at the same time.
• Do not change multiple layers before identifying the failing layer.
• Do not delete production data, grant broad permissions, or disable security controls as a first response.

Evidence quality

Auto-generated operator draft: includes issue-specific causes, commands, output branches, and unsafe-action warnings. Official-source links and real incident validation are queued for enrichment.

Commands to run first

grep -R "550" ./logs

dig example.com MX +short

dig example.com TXT +short

openssl s_client -starttls smtp -connect mail.example.com:587

grep -R "550" /var/log/mail*

dig TXT example.com +short

dig MX example.com +short

grep -R "spf\|dkim\|dmarc\|550\|554\|bounce" ./mail-logs ./logs

date -u && grep -R "ERROR\|WARN\|failed" ./logs | tail -n 50

Fix order

1. Record the full 550 message, failing URL, user, object ID, and latest change.
2. Collect issue-specific evidence for mail delivery failure caused by SPF/DKIM/DMARC alignment, MX routing, SMTP authentication, bounce policy, or blacklist reputation.
3. Compare the failing case with a successful case before editing settings.
4. If this is the first triage branch, Collect timestamp, route, request ID, user, and the first backend log line before editing configuration.
5. Re-check with the same command and URL, then record the normal output.

Actions by cause

• For 550, apply the fix only after reproducing the same condition and saving the before/after evidence for this exact code.
• Check SPF, DKIM, and DMARC results from the receiver's headers.
• Publish the exact DKIM selector used by the sending service.
• Align envelope, header From, and signing domains.
• Confirm MX and SMTP credentials belong to the active mail provider.
• Handle 550/554 bounce codes by policy reason, not by retrying blindly.
• For the first triage branch, Collect timestamp, route, request ID, user, and the first backend log line before editing configuration.

Evidence links

• Google Postmaster Tools official
• DMARC overview official

Verification metadata

• operator-draft
• official-reference-linked
• 2026-07-23

Update queue

• Review cadence
  weekly-source-review
• Next enrichment
  Add one official-source check and one real output example for Mail 550.

Environment-specific checks

• Shared hosting, proxies, VPNs, or CDN layers can change sender auth failure from receiver blocking results.
• Do not trust only the Mail UI; compare command output.
• Japanese hosting panels may show completion before DNS or SSL fully propagates.
• Test from both office and external networks.

Prevent it next time

• Store normal examples for SPF, DKIM, DMARC, MX, raw bounce.
• Add SPF include, DKIM selector, DMARC policy, reverse DNS to the release checklist.
• Keep recurring errors in the same note format.
• Split alerts by error rate, latency, certificates, disk, and permission changes.

Related notes

• Mail SPF DKIM DMARC authentication failure
• Mail SPF first triage response
• Mail DKIM first triage response
• Mail DMARC first triage response
• Mail 554 first triage response
• Mail bounce first triage response
• Mail blacklist first triage response
• Mail MX first triage response