FreeBSD 12 – sendmail queue grows because upstream DNS resolution times out

📖 ~1 min read

Table of contents

1. Symptom & Impact
2. Environment & Reproduction
3. Root Cause Analysis
4. Quick Triage
5. Step-by-Step Diagnosis
6. Solution – Primary Fix
7. Solution – Alternative Approaches
8. Verification & Acceptance Criteria
9. Rollback Plan
10. Prevention & Hardening
11. Related Errors & Cross-Refs
12. References & Further Reading

Symptom & Impact

Outbound mail delays increase and queue depth grows rapidly.

Environment & Reproduction

Seen when resolver path changes or upstream DNS latency spikes.

Root Cause Analysis

sendmail cannot resolve destination MX/A records in time.

Quick Triage

Validate resolver health and outbound SMTP network path.

Step-by-Step Diagnosis

Inspect maillog deferred errors and DNS response behavior.

Solution – Primary Fix

Correct resolver config, restart MTA, and force queue run.

Solution – Alternative Approaches

Deploy local caching resolver (unbound) for stability.

Verification & Acceptance Criteria

Queue drains and new deliveries complete within SLA.

Rollback Plan

Restore prior resolv.conf and disable local resolver if needed.

Prevention & Hardening

Monitor queue depth and DNS latency as core mail health metrics.

Related Errors & Cross-Refs

Can overlap with firewall restrictions on DNS or SMTP ports.

Related tutorial: View the step-by-step tutorial for FreeBSD 12.

View all FreeBSD 12 tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

sendmail operational guide and FreeBSD resolver docs.