Summary
STARTTLS negotiation failures, often resulting in the error 'connection died while negotiating STARTTLS TLS', arise from a complex interplay of network, configuration, and software-related factors. Diagnosing these issues often starts with manual testing using tools like SWAKS. Key contributing factors include network firewalls blocking port 25, incorrect or expired SSL/TLS certificates, incompatible cipher suites or TLS versions between client and server, misconfigured Postfix TLS settings, outdated OpenSSL libraries, and overly aggressive security settings. Resource exhaustion and improper implementation of the STARTTLS extension itself can also trigger failures. Addressing these problems requires a systematic approach, including verifying network connectivity, validating certificates, aligning configurations, updating software, reviewing security measures, and monitoring server resources.
Key findings
- Network Issues: Firewalls blocking port 25 can prevent successful STARTTLS negotiation.
- SSL/TLS Certificate Problems: Expired, mismatched hostname, or improperly configured intermediate certificates are frequent causes of failure.
- Cipher Suite/TLS Version Mismatch: Incompatible cipher suites or TLS versions between the client and server lead to negotiation errors.
- Postfix Configuration: Misconfigured Postfix TLS settings (e.g., `smtp_tls_security_level`) can disrupt the negotiation process.
- Outdated OpenSSL Libraries: Older OpenSSL versions may not support newer TLS protocols or cipher suites, causing negotiation to fail.
- Security Settings Interference: Overly aggressive security settings or IDS can block the STARTTLS handshake.
- Resource Exhaustion: High CPU usage or insufficient memory can lead to failures.
- Authentication Failures: Incompatible authentication mechanisms requested after STARTTLS can kill the connection.
- Faulty TLS System: The sending TLS system could be at fault, if successful TLS connection is achieved at the receiving end
- Testing Difficulties: The sending system could be negotiating with a key that the receiver does not like.
Key considerations
- Test with SWAKS: Use SWAKS for manual testing to obtain more detailed error information.
- Verify Network Connectivity: Ensure firewalls allow traffic on port 25 between the sending and receiving servers.
- Validate SSL/TLS Certificates: Check for certificate expiration, hostname mismatches, and proper intermediate certificate configuration.
- Align Cipher Suites and TLS Versions: Ensure compatibility between the client and server regarding supported cipher suites and TLS versions.
- Review Postfix Configuration: Verify the `smtp_tls_security_level` and other relevant TLS parameters in Postfix.
- Update OpenSSL: Keep OpenSSL libraries up-to-date to support modern TLS protocols.
- Examine Security Settings: Review firewall rules and IDS configurations for interference with the STARTTLS handshake.
- Monitor Server Resources: Ensure the server has sufficient resources (CPU, memory) to handle TLS negotiation.
- Verify Authentication Methods: Ensure the authentication methods are compatible with the destination receiving server.
- Faulty TLS system: The sending TLS system may need an upgrade to be compliant with the receiving server.
What email marketers say
10 marketer opinions
STARTTLS negotiation failures, indicated by the error 'connection died while negotiating STARTTLS TLS', stem from a variety of issues. These include configuration problems on either the client or server side, network disruptions, and security settings. Common causes include certificate mismatches, outdated libraries, unsupported TLS versions or cipher suites, resource exhaustion, and overly aggressive security measures. Addressing this error requires a multi-faceted approach, including checking certificate validity, updating software, verifying network connectivity, and ensuring compatible configurations.
Key opinions
- Certificate Problems: Certificate issues, such as mismatches between the hostname and the certificate's subject name, expired certificates, or improperly configured intermediate certificates, are frequent causes of STARTTLS failures.
- Configuration Mismatches: Incompatible TLS versions, cipher suites, or authentication mechanisms between the client and server can prevent successful STARTTLS negotiation.
- Network Issues: Temporary network glitches, intermittent DNS resolution problems, or firewall restrictions can disrupt the STARTTLS handshake.
- Software Issues: Outdated OpenSSL libraries on the sending server can be incompatible with newer TLS protocols, causing negotiation failures.
- Security Settings: Overly aggressive security settings or intrusion detection systems (IDS) can interfere with the STARTTLS handshake.
- Resource Exhaustion: High CPU usage or insufficient memory on the server can lead to STARTTLS failures due to resource exhaustion.
Key considerations
- Certificate Validation: Ensure the SSL/TLS certificate is valid, not expired, and correctly configured with a matching hostname. Verify the certificate chain is properly set up.
- Configuration Review: Check and align TLS versions, cipher suites, and authentication mechanisms supported by both the sending and receiving servers.
- Network Troubleshooting: Diagnose and resolve network connectivity issues, including firewall rules and DNS resolution problems. Consider restarting the mail server.
- Software Updates: Update OpenSSL and other relevant software libraries to the latest versions to ensure compatibility with modern TLS protocols.
- Security Audit: Review security settings and IDS rules to ensure they are not interfering with the STARTTLS handshake. Adjust settings as needed.
- Resource Monitoring: Monitor server resources (CPU, memory) and optimize performance to prevent resource exhaustion during STARTTLS negotiation.
Marketer view
Email marketer from ServerFault mentions that an outdated OpenSSL library on the sending server can cause STARTTLS failures, especially when communicating with servers using newer TLS protocols. Updating OpenSSL may resolve the issue.
24 Jan 2024 - ServerFault
Marketer view
Email marketer from Email Admins Group shares that if the receiving server requires a specific authentication mechanism after STARTTLS, and the sending server doesn't support it or isn't configured correctly, the connection can die during negotiation. Ensuring authentication methods are compatible is key.
5 Dec 2024 - Email Admins Group
What the experts say
5 expert opinions
STARTTLS negotiation failures can be diagnosed using tools like SWAKS for manual connection testing to reveal clearer error messages. These failures might be due to issues with the sender's TLS system, an incompatible key being negotiated, or underlying problems with the SSL/TLS certificate such as expiration or hostname mismatches.
Key opinions
- Testing with SWAKS: Using SWAKS to manually test the connection can provide more detailed error messages to help diagnose STARTTLS negotiation failures.
- TLS System Issues: The sender's TLS system might be faulty or incompatible with the recipient's requirements, leading to negotiation failures.
- Key Negotiation Problems: The sender might be attempting to negotiate with a key that the recipient doesn't accept, causing the negotiation to fail.
- Certificate Problems: Issues with the SSL/TLS certificate, such as an expired certificate or a mismatch between the hostname and the certificate's subject name, can cause STARTTLS failures.
Key considerations
- Manual Testing: Utilize SWAKS to manually test the connection and gather more detailed error information.
- TLS System Check: Examine the sender's TLS system for faults or incompatibilities, and update or reconfigure as needed.
- Key Compatibility: Ensure the key being negotiated is compatible with the recipient's requirements.
- Certificate Verification: Check the SSL/TLS certificate for expiration, hostname mismatches, and other potential issues. Renew or reconfigure the certificate as necessary.
Expert view
Expert from Word to the Wise explains that one potential cause of STARTTLS negotiation failures is a problem with the SSL/TLS certificate, such as an expired certificate or a mismatch between the hostname and the certificate's subject name. The fix may involve renewing the certificate, ensuring correct hostname configuration, or checking certificate chain issues.
3 Sep 2023 - Word to the Wise
Expert view
Expert from Email Geeks suggests the issue might be that the sender is negotiating with a key the recipient doesn’t like.
13 May 2025 - Email Geeks
What the documentation says
5 technical articles
STARTTLS negotiation failures are often attributed to network issues like firewalls blocking port 25, incorrect SSL configurations, incompatible cipher suites, misconfigured Postfix TLS settings, outdated OpenSSL versions with known issues, or improper implementation of the STARTTLS extension itself. Resolving these issues involves verifying network connectivity, validating SSL certificates, ensuring cipher suite compatibility, correctly configuring Postfix settings, updating OpenSSL, and adhering to the STARTTLS extension specifications.
Key findings
- Network Blockage: Firewalls blocking the STARTTLS handshake on port 25 are a common cause.
- SSL Misconfiguration: Incorrect SSL configuration or invalid SSL certificates can lead to failures.
- Cipher Suite Mismatch: Incompatible cipher suites between the client and server cause negotiation problems.
- Postfix TLS Misconfiguration: Incorrect settings in Postfix, specifically related to TLS security levels, can result in errors.
- Outdated OpenSSL: Specific OpenSSL versions may have known issues related to TLS negotiation with certain cipher suites or protocol versions.
- STARTTLS Implementation Issues: Improper implementation of the STARTTLS extension (RFC 3207) can cause failures.
Key considerations
- Check Firewall Rules: Ensure that firewall rules are not blocking the STARTTLS handshake on port 25 between sending and receiving servers.
- Validate SSL Certificate: Verify that the server's SSL certificate is valid and that cipher suites are compatible between the client and server.
- Review Postfix Configuration: Check the `smtp_tls_security_level` and related parameters in the Postfix configuration to ensure they are correctly set.
- Update OpenSSL: Check and update the OpenSSL version to address known issues related to TLS negotiation.
- Implement STARTTLS Correctly: Ensure proper implementation of the STARTTLS extension as specified in RFC 3207.
Technical article
Documentation from OpenSSL.org notes that specific OpenSSL versions have known issues related to TLS negotiation with certain cipher suites or protocol versions. The documentation suggests checking the OpenSSL version and updating it if necessary.
31 Jul 2024 - OpenSSL.org
Technical article
Documentation from cPanel Documentation suggests the problem may be related to an incorrect SSL configuration on the server, or a mismatch in supported ciphers. It recommends ensuring that the server's SSL certificate is valid and that the cipher suites are compatible between the client and server.
11 Sep 2023 - cPanel Documentation
