Mail MX proxy versus origin split response

When this happens

Use this when a cdn, load balancer, reverse proxy, or waf sits in front of the service. Do not stop at the screen message; validate whether the edge/proxy or the origin server creates the failure first.

Symptom checklist

• MX 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

• MX specifically changes the investigation surface for Mail: verify the exact failing object, route, user, and timestamp before applying the broader pattern.
• The authoritative zone does not contain the record expected by the application.
• Recursive resolvers still hold an older value because TTL or negative cache has not expired.
• The record exists in a different DNS provider than the active nameservers.
• Policy records such as CAA, MX, SPF, or TXT are syntactically present but semantically wrong.
• The service target was renamed while DNS still points to the old endpoint.
• For the proxy versus origin split case, the first useful clue is whether the edge/proxy or the origin server creates the failure.

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: DNS resolution failure caused by missing records, wrong authority, stale TTL, policy records, or service target mismatch.
4. Classify this as proxy versus origin split: whether the edge/proxy or the origin server creates the failure.
5. Capture current values before changing configuration.

First evidence

Treat MX as a proxy versus origin split case. First collect evidence for whether the edge/proxy or the origin server creates the failure.

Output examples

Normal output

Direct origin and routed proxy return equivalent status and headers.

Failing output

Origin succeeds but proxy fails, or proxy succeeds while direct origin exposes the real backend error.

Output-to-action branches

• A CDN, load balancer, reverse proxy, or WAF sits in front of the service.
  Test direct origin and routed traffic separately, then change only the layer that differs.
• The working and failing outputs differ.
  Act on the differing layer first: For MX, 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 assume the CDN error code is the application root cause.
• 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 "MX" ./logs

dig example.com MX +short

dig example.com TXT +short

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

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

dig +trace example.com

dig @1.1.1.1 example.com

dig NS example.com +short

dig TXT example.com +short

curl -Iv https://example.com --resolve example.com:443:ORIGIN_IP

Fix order

1. Record the full MX message, failing URL, user, object ID, and latest change.
2. Collect issue-specific evidence for DNS resolution failure caused by missing records, wrong authority, stale TTL, policy records, or service target mismatch.
3. Compare the failing case with a successful case before editing settings.
4. If this is the proxy versus origin split branch, Test direct origin and routed traffic separately, then change only the layer that differs.
5. Re-check with the same command and URL, then record the normal output.

Actions by cause

• For MX, apply the fix only after reproducing the same condition and saving the before/after evidence for this exact code.
• Check authoritative nameservers first, then compare public resolvers.
• Correct the record in the active zone, not in an inactive provider account.
• Wait out negative cache when NXDOMAIN was recently published.
• Validate TXT, MX, CAA, and alias targets with service-specific tools.
• Keep the old and new DNS values in the change note until propagation finishes.
• For the proxy versus origin split branch, Test direct origin and routed traffic separately, then change only the layer that differs.

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 MX.

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