Summary
SSL/TLS key size errors in email SMTP transactions occur when the security keys used by either the sending or receiving server during the Transport Layer Security (TLS) handshake are considered too weak or incompatible by the other party. This issue primarily impacts the secure establishment of the SMTP connection, preventing email delivery.
Key findings
- Handshake failure: The core of the problem lies in the TLS handshake process, where the two servers cannot agree on a mutually acceptable encryption key size. One server may require a minimum key size that the other server's certificate does not meet.
- Security standards: Older, weaker key sizes, such as 1024-bit RSA keys, are increasingly deprecated or outright rejected by modern mail servers due to evolving cryptographic security standards. Many systems now enforce a minimum of 2048-bit RSA or stronger equivalent algorithms.
- Impact on deliverability: When a key size error occurs, the SMTP transaction fails, leading to undelivered emails, bounced messages, and potential email deliverability issues.
- Misdiagnosis: These errors can sometimes be confused with issues related to DKIM (DomainKeys Identified Mail) records because both involve cryptographic keys. However, SSL/TLS errors relate to the transport layer encryption, while DKIM is an email authentication method that signs the email content.
Key considerations
- Upgrade certificates: Ensure your mail server's SSL/TLS certificates (and their associated private keys) meet current industry standards, typically 2048-bit RSA or higher. Regularly review certificate expiration dates.
- Server configuration: Verify that your mail server's SMTP service is configured to support robust TLS protocols and cipher suites with adequate key sizes. Outdated configurations can lead to rejection by stricter receiving servers.
- Monitor delivery logs: Examine SMTP transaction logs for specific error messages related to TLS handshake failures or key size mismatches. These logs often provide clues about which server (sending or receiving) is initiating the rejection.
- Stay informed: Security standards for encryption evolve. Staying updated on best practices for TLS configuration, as detailed by organizations like the Internet Society, is crucial for maintaining optimal email deliverability.
What email marketers say
Email marketers often encounter SSL/TLS key size errors indirectly, typically as delivery failures or unexplained bounces, rather than explicit technical warnings. Their focus is generally on ensuring messages reach the inbox, making these underlying technical issues a significant, albeit often hidden, impediment to their campaigns.
Key opinions
- Impact on campaigns: Undelivered emails due to TLS errors directly affect campaign performance metrics, such as open rates and click-through rates, leading to wasted effort and missed opportunities.
- Confusion with other errors: Marketers may confuse these errors with other email authentication failures like SPF or DKIM, as all can manifest as delivery problems. Understanding the specific nature of the error is crucial for proper troubleshooting.
- Dependency on IT/Ops: Resolving key size errors usually requires server-side configuration changes or certificate updates, which are typically beyond the scope of a marketer's direct control and necessitate collaboration with IT or DevOps teams.
- Hidden deliverability blocks: Because TLS errors can silently prevent messages from reaching the inbox, marketers might only discover them through bounce reports or a general decline in email deliverability rates.
Key considerations
- Proactive monitoring: Implement tools that provide detailed deliverability insights beyond simple bounce rates, ideally offering visibility into TLS connection failures.
- Communicate with IT: Maintain an open dialogue with technical teams about email infrastructure health, ensuring that security configurations like SSL/TLS certificates are regularly reviewed and updated.
- Use testing tools: Utilize an email deliverability tester to diagnose connection issues and identify potential TLS problems before they impact a large audience.
- Understand basics: While not requiring deep technical expertise, a basic understanding of how TLS works for email (like its role in encrypting the SMTP connection) can help marketers better articulate issues to technical support. More information on common TLS errors can be found on SE Ranking's blog.
Email marketer from Email Geeks indicates they are currently using a 1024-bit key and are investigating if this is the source of the problem, particularly if another server expects a larger key size for a secure connection.
A marketer from Quora highlights that an SSL certificate error often indicates issues like an incorrect system date, a domain name mismatching the certificate, or the certificate being self-issued rather than from a trusted authority, all of which can affect secure email transmission.
What the experts say
Email deliverability experts highlight that SSL/TLS key size errors are a fundamental issue of cryptographic strength and server compatibility. They emphasize that such errors prevent the secure delivery of emails, often pointing to outdated server configurations or non-compliant certificates as primary causes. Proper configuration is essential for modern email ecosystems.
Key opinions
- Not DKIM: Experts clarify that key size errors during SMTP transactions are distinct from DKIM verification issues. These errors specifically pertain to the secure channel (TLS) established between two mail servers, independent of email content signing.
- SMTP encryption: The error directly impacts the encryption of the actual SMTP transaction, functioning like HTTPS for web browsing, but applied to email transport.
- Key length inadequacy: The core of the problem is that one of the cryptographic keys involved in the TLS negotiation is too short, failing to meet the minimum security requirements of the other server.
- Server-side rejection: Typically, such an error indicates the recipient server is rejecting the connection because the sending server's key, or vice-versa, does not meet its security policy, highlighting the need for both sides to maintain up-to-date security.
Key considerations
- Certificate management: Regularly audit and update SSL/TLS certificates on all mail servers to ensure they use strong, current key sizes (e.g., 2048-bit RSA or ECDSA equivalents) and are not expired.
- Protocol support: Ensure mail servers support modern TLS versions (TLS 1.2 or TLS 1.3) and have strong cipher suites enabled, while disabling weaker, older protocols.
- Comprehensive deliverability audits: Go beyond basic email authentication checks like SPF, DKIM, and DMARC to include regular assessments of your SMTP server's TLS configuration. This is part of maintaining advanced email authentication.
- Troubleshooting methodology: When diagnosing deliverability issues, consider TLS key size or protocol mismatches, especially if the problem is specific to certain receiving domains or involves encrypted connections. This often requires checking server logs for detailed error codes, as suggested by The SSL Store.
Expert from Email Geeks explains that the error message is specifically related to SSL/TLS, not DKIM, clarifying that it concerns the encryption handshake between the sending and receiving servers during the SMTP transaction.
An expert from SpamResource states that weak or unsupported cipher suites and outdated SSL/TLS versions are common culprits for handshake failures, urging senders to update their server configurations to meet modern security expectations.
What the documentation says
Technical documentation consistently emphasizes that strong cryptographic key sizes are fundamental for secure communication, including SMTP transactions. As standards evolve, older, weaker key sizes are actively phased out, and systems are designed to reject connections that do not meet current security benchmarks. This ensures forward secrecy and protects data integrity during email transfer.
Key findings
- RFC standards: RFCs defining TLS protocols (e.g., RFC 8446 for TLS 1.3) specify the handshake process and the cryptographic algorithms to be used, implicitly guiding acceptable key sizes. Deviations lead to connection failures.
- Protocol evolution: The evolution from SSL to TLS (and subsequent TLS versions like 1.2 and 1.3) includes strengthened requirements for key exchange mechanisms and cipher suites, leading to the deprecation of smaller key sizes.
- Interoperability: To ensure interoperability and security, both communicating parties must support compatible TLS versions and key sizes. A mismatch in capabilities will result in a failed connection.
- Certificate validity: Beyond key size, documentation highlights that certificates must be valid, unexpired, and issued by a trusted Certificate Authority (CA) to facilitate a successful TLS handshake.
Key considerations
- Adherence to current RFCs: Mail server administrators should configure their systems to adhere to the latest RFCs for TLS, such as RFC 8446 (TLS 1.3), to ensure robust encryption and avoid compatibility issues.
- Regular updates: Server software and cryptographic libraries (like OpenSSL) must be regularly updated to support the latest security protocols and key sizes. This is critical for preventing email delivery failures.
- Strong cipher suites: Configuration should prioritize strong cipher suites that enforce robust key exchange algorithms and sufficient key lengths, disabling any weak or deprecated options.
- Authentication methods: While distinct, proper TLS configuration complements email authentication protocols such as DMARC, SPF, and DKIM, collectively ensuring both secure transport and message authenticity.
Sectigo Official documentation notes that certificate issues, including revoked, inactive, or expired certificates, are common causes of TLS errors, implying that key size, though not explicitly mentioned here, is a critical component of certificate validity.
The IETF Datatracker, via RFC 8446 for TLS 1.3, specifies robust cryptographic requirements for establishing secure communication over the Internet, effectively making older, weaker key sizes non-compliant with the latest protocol version.
