Postfix mail queue stuck and undelivered

📖 ~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

Alert emails and automated notifications are delayed or never delivered.

Environment & Reproduction

RHEL 7 host running postfix with relayhost configuration and restricted outbound SMTP.

Root Cause Analysis

Relay DNS failure, authentication issues, or firewall block on SMTP destination port.

Quick Triage

Check mailq size, systemctl status postfix, and connectivity to relay endpoint.

Step-by-Step Diagnosis

Review /var/log/maillog, run postconf -n, and test route with telnet or openssl s_client.

Solution – Primary Fix

Correct relayhost and credentials, open required port in firewalld/network ACL, then postqueue -f.

Solution – Alternative Approaches

Switch to local smarthost gateway or API-based notification mechanism for critical alerts.

Verification & Acceptance Criteria

mailq drains to near zero and test message reaches target mailbox within SLA.

Rollback Plan

Reapply previous postfix main.cf and restart postfix service if new relay config fails.

Prevention & Hardening

Monitor queue depth and relay health with proactive alert thresholds.

Related Errors & Cross-Refs

deferred: connect timed out, SASL authentication failed, host not found.

Related tutorial: View the step-by-step tutorial for rhel-7.

View all rhel-7 tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Postfix documentation, RHEL mail services guide, SMTP relay hardening practices.