FreeBSD 12 – Common Problem 015 – Sendmail Queue Builds Up and Mail Stalls

📖 ~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

System alerts and transactional emails are delayed or not delivered.

Environment & Reproduction

Queue growth appears after DNS or relay host changes.

Root Cause Analysis

Unreachable SMTP relay, blocked port 25/587, or bad masquerade settings.

Quick Triage

Check queue depth and daemon status.

mailq | head
service sendmail status
netstat -an | grep ':25'

Step-by-Step Diagnosis

Review mail logs and validate relay name resolution.

tail -n 80 /var/log/maillog
host relay.example.net
traceroute relay.example.net

Solution – Primary Fix

Correct relay config and flush queued mail.

service sendmail restart
sendmail -q -v
mailq | head

Solution – Alternative Approaches

Use `dma` or external MTA relay service for simpler host-level alerting needs.

Verification & Acceptance Criteria

Queue drains to normal baseline and test email reaches target mailbox.

Rollback Plan

Reinstate previous sendmail config from backup and restart daemon.

Prevention & Hardening

Monitor queue length and configure fallback relay for transient upstream failures.

Related Errors & Cross-Refs

`Deferred: Connection timed out`, `Host unknown`, backlog in `mailq`.

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(8)`, FreeBSD mail configuration guide.