FreeBSD 15 sendmail Queue Growth Due to DNS Lookup Failures

📖 ~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

Outgoing mail delays escalate and queue size grows rapidly.

Environment & Reproduction

Frequently observed when resolver settings are changed during maintenance.

mailq

Root Cause Analysis

sendmail cannot resolve destination MX records, so deliveries are deferred repeatedly.

Quick Triage

Test DNS lookups for representative destination domains.

host -t mx example.com

Step-by-Step Diagnosis

Inspect sendmail logs and resolver configuration for timeout patterns.

grep -i sendmail /var/log/maillog | tail -n 100

Solution – Primary Fix

Repair DNS resolver path and trigger queue retry processing.

sudo service sendmail restart && sudo sendmail -q

Solution – Alternative Approaches

Relay mail through upstream smarthost while local DNS is restored.

Verification & Acceptance Criteria

Queue depth declines and new messages deliver without extended defer.

Rollback Plan

Revert resolver changes and restore prior mail relay settings.

Prevention & Hardening

Monitor mail queue depth and DNS latency with proactive alert thresholds.

Related Errors & Cross-Refs

Can align with network ACL changes affecting outbound DNS.

Related tutorial: View the step-by-step tutorial for freebsd-15.

View all freebsd-15 tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

FreeBSD sendmail operations and troubleshooting references.