FreeBSD 15 – 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 email delays increase and queue files accumulate until storage or SLA limits are hit.

Environment & Reproduction

Appears when resolver path changes or upstream DNS latency spikes.

mailq | head -n 40
service sendmail status
cat /etc/resolv.conf

Root Cause Analysis

sendmail cannot resolve MX/A records in time, deferring deliveries repeatedly.

Quick Triage

Verify resolver health and outbound SMTP reachability.

drill gmail.com mx
fetch -o /dev/null https://1.1.1.1 || true
nc -vz smtp.gmail.com 25 || true

Step-by-Step Diagnosis

Inspect queue diagnostics and resolver timeout behavior.

grep -i 'deferred|host name lookup failure' /var/log/maillog | tail -n 80
truss -p $(pgrep sendmail | head -n1) 2>/dev/null | head -n 40

Solution – Primary Fix

Correct resolver configuration, restart MTA, then force queue run.

vi /etc/resolv.conf
service sendmail restart
/usr/sbin/sendmail -q -v

Solution – Alternative Approaches

Use local unbound for resilient caching and reduced external DNS dependency.

pkg install -y unbound
service local_unbound enable
service local_unbound start

Verification & Acceptance Criteria

Queue drains steadily and new messages deliver within expected interval.

mailq | head -n 20
grep -i 'stat=Sent' /var/log/maillog | tail -n 20

Rollback Plan

Restore prior resolver config and disable local cache if it introduces failures.

cp /root/backup/resolv.conf /etc/resolv.conf
service local_unbound stop || true

Prevention & Hardening

Monitor queue depth and DNS latency as first-class mail health indicators.

mailq | wc -l
drill . ns

Related Errors & Cross-Refs

May overlap with outbound firewall filters on port 53 or 25.

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

sendmail operation guide and FreeBSD resolver subsystem documentation.