Mail SMTP auth specific path failure response

When this happens

Use this when only one page, endpoint, static asset, or api route fails. Do not stop at the screen message; validate the route, rewrite rule, API path, host header, and upstream mapping first.

Symptom checklist

• SMTP auth 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

• SMTP auth specifically changes the investigation surface for Mail: verify the exact failing object, route, user, and timestamp before applying the broader pattern.
• The credential exists but no longer matches the redirect, session, cookie, or token validation rules.
• SameSite, Secure, or domain attributes prevent the browser from sending the cookie.
• A CSRF state value is generated under one origin and posted to another.
• OAuth callback URLs differ between staging, production, and provider settings.
• A rotated SMTP or API credential is not deployed to every runtime.
• For the specific path failure case, the first useful clue is the route, rewrite rule, API path, host header, and upstream mapping.

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: authentication failure caused by expired token, redirect mismatch, cookie policy, CSRF state, or SMTP credential state.
4. Classify this as specific path failure: the route, rewrite rule, API path, host header, and upstream mapping.
5. Capture current values before changing configuration.

First evidence

Treat SMTP auth as a specific path failure case. First collect evidence for the route, rewrite rule, API path, host header, and upstream mapping.

Output examples

Normal output

Working and failing paths use the intended host, status, and upstream.

Failing output

Only one route changes status, redirect target, content type, or upstream.

Output-to-action branches

• Only one page, endpoint, static asset, or API route fails.
  Compare a working path and failing path, then fix routing, rewrite, upload limit, or upstream mapping.
• The working and failing outputs differ.
  Act on the differing layer first: For SMTP auth, 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 clear every cache before proving the route mapping.
• 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 "SMTP auth" ./logs

dig example.com MX +short

dig example.com TXT +short

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

grep -R "SMTP auth" /var/log/mail*

curl -I https://example.com/login

grep -R "csrf\|oauth\|jwt\|samesite\|auth" ./logs

printenv | grep -i "AUTH\|OAUTH\|SESSION"

curl -I https://example.com/failing-path && curl -I https://example.com/working-path

Fix order

1. Record the full SMTP auth message, failing URL, user, object ID, and latest change.
2. Collect issue-specific evidence for authentication failure caused by expired token, redirect mismatch, cookie policy, CSRF state, or SMTP credential state.
3. Compare the failing case with a successful case before editing settings.
4. If this is the specific path failure branch, Compare a working path and failing path, then fix routing, rewrite, upload limit, or upstream mapping.
5. Re-check with the same command and URL, then record the normal output.

Actions by cause

• For SMTP auth, apply the fix only after reproducing the same condition and saving the before/after evidence for this exact code.
• Trace one login or auth attempt from redirect to callback to session creation.
• Align cookie domain, Secure, SameSite, and proxy HTTPS headers.
• Update OAuth callback allowlists for the exact production URL.
• Rotate credentials once and redeploy all consumers together.
• Log token expiry and issuer/audience mismatch without exposing secrets.
• For the specific path failure branch, Compare a working path and failing path, then fix routing, rewrite, upload limit, or upstream mapping.

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 SMTP auth.

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 550 first triage response
• Mail 554 first triage response
• Mail bounce first triage response
• Mail blacklist first triage response