CentOS Stream 10 – Common Problem 247 – Diagnosis and Fix

📖 ~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

Senders connecting to Postfix receive 530 or TLS handshake failures, blocking inbound mail flow.

Environment & Reproduction

Happens after hardening smtpd_tls_security_level=encrypt or restricting cipher list too far.

Root Cause Analysis

Mandatory TLS plus crypto-policies removes ciphers needed by older sending MTAs.

Quick Triage

Inspect maillog for warning: SSL_accept error and openssl s_client output to the listener.

Step-by-Step Diagnosis

Run: sudo postconf -n | grep tls; sudo journalctl -u postfix -e; openssl s_client -starttls smtp -connect host:25.

Solution – Primary Fix

Relax to smtpd_tls_security_level = may and align smtpd_tls_mandatory_ciphers with crypto-policies.

Solution – Alternative Approaches

Implement MTA-STS and DANE so modern senders enforce TLS while legacy senders fall back gracefully.

Verification & Acceptance Criteria

Test senders connect over TLS 1.2 or higher and queue drains without TLS-related deferrals.

Rollback Plan

Restore previous main.cf from backup if mail volume drops sharply after the change.

Prevention & Hardening

Track sender TLS posture with smtpd_tls_loglevel = 1 and review per-domain failure rates.

Related Errors & Cross-Refs

Related to header_checks rejecting clear-text submission and Dovecot SASL pass-through issues.

Related tutorial: View the step-by-step tutorial for centos-stream-10.

View all centos-stream-10 tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Postfix TLS_README and Red Hat mail server documentation.