Mail Not Flowing
Zero delivery success, complete connector failure, or DNS/routing block.
Diagnostic entry point for total mail loss scenarios.
Exchange Online Mail Flow Troubleshooting
Deep diagnostic procedures for inbound/outbound mail failures, routing issues, NDRs, and delivery delays. Includes safe rollback and escalation criteria. Use message trace for evidence gathering.
Mail Flow Problem Categories
Delayed Email Delivery
Mail queuing, queue length monitoring, and delivery delays exceeding SLA.
Prioritized queue diagnostics and relief procedures.
NDR 550 5.1.1 (Recipient Rejected)
Invalid recipient, address rewrite failure, or directory sync issue.
Long-tail error code targeting and root cause mapping.
NDR 5.5.4 (Service Unavailable)
Temporary service unavailability, connector overload, or transport rule blocking.
Decision tree for transient vs. permanent failures.
Throttling Detection & Relief
High-volume mail blocking, EOP throttling, and bulk operation limits.
Identifying throttling patterns and safe escalation.
Connector & Routing Issues
Inbound connector configuration, routing rules, and send connector failures.
Safe configuration audit and rollback procedures.
Quick Diagnostic Checklist
- Confirm scope: Single recipient, tenant domain, or all traffic?
- Check message trace: Is mail queued, rejected, or silent dropped?
- Verify connectors: Inbound/outbound connector health and queue depth
- Review transport rules: Are rules rejecting or redirecting?
- Check DLP/journaling: Silent drops due to policy or journal failures
- Validate DNS: MX, SPF, DKIM, and DMARC records
- Assess for throttling: Check throttling policy and queue monitoring
- Review recent changes: Connector, rule, or security policy modifications
Root Cause Patterns (By Frequency)
| Pattern | Symptom | Fix Type |
|---|---|---|
| Connector down/misconfigured | Zero inbound, queued outbound | Restart/reconfigure connector |
| Directory sync lag | NDR 550 5.1.1 (invalid recipient) | Force sync, validate mail attributes |
| Transport rule silently drops | Mail missing, no NDR | Audit and disable rule |
| DLP/journal queue full | Delays, eventual delivery or NDR | Clear journal queue, investigate journal target |
| Throttling policy triggered | Intermittent NDR 5.5.4 | Adjust throttling policy, reduce rate |
When to Escalate to Microsoft
- Connector shows healthy in admin center but mail still not flowing
- Message trace shows accepted by EOP but never delivered to tenant
- Throttling persists after policy adjustment and queue is clear
- Silent drops continue after transport rules audit and DLP review
- External mail flows fine but tenant-to-tenant fails (multi-tenant federation issue)
Full Runbooks
Step-by-step remediation with rollback procedures.