TROUBLESHOOTING

Start with the symptom

You should not need to know which protocol is broken before you can troubleshoot it.

Mail bounced

Troubleshoot bounces · Enhanced status codes · Investigate 550/5.7.1

Mail is delayed

Troubleshoot delays · Analyze a delivery delay · Queue incident runbook

Mail goes to spam

Troubleshoot spam placement · Reputation signals · Build a deliverability baseline

SPF, DKIM, or DMARC fails

SPF · DKIM · DMARC · SPF passes but DMARC fails · DKIM passes but DMARC fails

Recipient is not receiving mail

No MX · Trace a delayed message · Microsoft 365 message trace · Google Workspace Email Log Search

TLS or secure transport fails

SMTP TLS · MTA-STS · TLS-RPT · Check an SMTP certificate

Mailbox or account looks compromised

Investigate mailbox compromise · Audit forwarding · Respond to BEC

Configuration changed and mail broke

Validate a DNS change · Verify a cutover · Email change management