What causes SSL/TLS key size errors in email SMTP transactions?
Michael Ko
Co-founder & CEO, Suped
Published 5 Jun 2025
Updated 19 Aug 2025
8 min read
Email is a cornerstone of modern communication, but its security relies heavily on robust encryption protocols. When you send an email, it often travels across various networks and servers before reaching its destination. Transport Layer Security (TLS), the successor to SSL, ensures this journey is secure, acting like a digital bodyguard for your messages during SMTP (Simple Mail Transfer Protocol) transactions.
However, sometimes this security handshake fails, leading to frustrating delivery issues. One specific problem that can arise is an SSL/TLS key size error during an email SMTP transaction. This error indicates a mismatch or inadequacy in the cryptographic keys used by the sending and receiving mail servers, preventing a secure connection from being established.
How SSL/TLS secures email
At its core, TLS encryption for email ensures privacy and data integrity between mail servers, much like Transport Layer Security (HTTPS) does for websites. When one mail server tries to communicate with another, they perform a TLS handshake. This is a sequence of steps where they agree on the encryption methods and securely exchange cryptographic keys. This process guarantees that the data transferred, including your email content, remains confidential and cannot be tampered with.
The role of cryptographic keys
A cryptographic key is a string of bits used by an encryption algorithm to transform plain text into ciphertext and vice versa. The strength of the encryption directly correlates with the key's length, or "key size." Longer keys generally mean more secure encryption, as they are harder to crack through brute force. Common key sizes include 1024-bit, 2048-bit, and 4096-bit RSA keys.
For email, this often involves opportunistic TLS, where servers attempt to upgrade a plain text SMTP connection to an encrypted one using the STARTTLS command. If both servers support TLS, they proceed with the handshake. If one server's security parameters, such as the minimum acceptable key size, don't align with the other's, the handshake fails, resulting in a key size error. This is also a common reason for STARTTLS negotiation failing.
What causes key size mismatches?
The most frequent cause of SSL/TLS key size errors is a mismatch in security requirements between the communicating mail servers. Modern email servers are increasingly configured to enforce higher security standards, including minimum key lengths for TLS certificates. If a sending server attempts to establish a connection with a certificate using a key size below this minimum, the receiving server will reject the connection, citing a key size error.
Outdated server configurations
Many older mail servers or misconfigured systems might still be using outdated certificates with key sizes of 1024 bits or less. While these might have been considered acceptable in the past, they are now deemed insecure by most modern standards and email providers. For instance, the U.S. General Services Administration recommends TLS server certificates using RSA keys have a minimum size of 2048 bits. When such an outdated certificate encounters a server enforcing these newer, stricter policies, the key size error occurs.
Best practice for key sizes
To ensure maximum compatibility and security, always aim to use TLS certificates with a minimum key size of 2048 bits for RSA keys, and preferably 4096 bits where supported. This aligns with current industry security recommendations and reduces the likelihood of key size rejection errors. Modern services like Microsoft 365 and Gmailstrictly enforce these security measures.
Impact on email transactions
When an SSL/TLS key size error occurs, the immediate consequence is a failed email delivery. The SMTP transaction between the two mail servers is interrupted, and the email simply won't reach its intended recipient. This can manifest as bounce messages in your mail logs, indicating a TLS handshake failure or an inability to establish a secure channel.
Reputation and blocklisting concerns
Consistent failures in establishing secure connections can negatively impact your sender reputation. ISPs and email providers track various metrics, including the success rate of TLS connections. A high rate of TLS errors, even if due to key size issues, can signal to receivers that your sending infrastructure is unreliable or insecure. This can lead to your emails being flagged as suspicious, routed to the spam folder, or even result in your sending IP address or domain being added to a blocklist (or blacklist). Regular blocklist checking is essential.
Older email systems
Often configured with outdated TLS certificates and smaller key sizes (e.g., 1024-bit RSA).
May not support modern TLS versions (TLS 1.2 or 1.3), leading to handshake failures when connecting to stricter servers.
Modern email systems
Require larger key sizes (e.g., 2048-bit or 4096-bit RSA) for enhanced security.
Configured to use the latest TLS protocol versions and strong cipher suites.
Troubleshooting and prevention
Troubleshooting key size errors requires examining your mail server logs for specific error messages related to TLS or SSL handshake failures. These logs often provide details on why the connection was rejected, sometimes explicitly mentioning key too small or insufficient key length. If you have access to the remote server's logs, they might offer more clues.
Verifying your certificate
You can use tools like OpenSSL to inspect your server's TLS certificate and determine its key size. This command connects to your SMTP port and displays certificate information:
The output will include details about the public key, including its length. If the key size is 1024 bits or less, consider upgrading your certificate. This process is similar to how you might fix invalid RSA public key errors in DKIM records, which also rely on cryptographic keys and proper sizing.
Solutions and prevention
Upgrade certificates: Replace any TLS certificates with key sizes below 2048 bits with stronger ones. This is the most effective solution.
Server configuration: Ensure your mail server is configured to support modern TLS versions (1.2 or 1.3) and strong cipher suites. Similarly, if you are the recipient of emails encountering these errors, check your server's settings to ensure it can accept connections with older, albeit less secure, key sizes if absolutely necessary for specific senders (though this is not recommended).
Regular monitoring: Implement robust email deliverability monitoring to catch TLS errors quickly. This proactive approach helps maintain a healthy sending reputation and ensures your emails consistently reach the inbox.
Views from the trenches
Expert from Email Geeks says: The issue often arises when one server's certificate key size is smaller than the minimum required by the other server.
Expert from Email Geeks says: SSL/TLS key size errors pertain specifically to the encryption of the SMTP transaction, not DKIM authentication.
Expert from Email Geeks says: During an SMTP transaction, sending and receiving servers negotiate the encryption protocols and key sizes for secure communication.
Expert from Email Geeks says: An insufficient key length on either side of the connection can cause a key size error.
Expert from Email Geeks says: If key size errors are observed from a single source, it often indicates your system's stringent security requirements are rejecting a peer's weaker key.
Marketer from Email Geeks says: I was using a 1024-bit key, which caused issues with some receiving servers that required larger key sizes for their connections.
Best practices
Regularly audit all TLS certificates used for email, ensuring they meet current industry standards for key length.
Prioritize the use of 2048-bit or higher RSA keys for all SMTP TLS certificates to maximize compatibility and security.
Configure mail servers to support the latest TLS protocol versions (TLS 1.2 and 1.3) and strong cipher suites.
Common pitfalls
Using outdated 1024-bit TLS certificates that are no longer trusted by many modern mail servers.
Ignoring TLS handshake errors in mail logs, which can lead to silent email delivery failures and reputation damage.
Failing to renew TLS certificates before they expire, causing immediate service disruptions.
Expert tips
Proactively check the TLS configurations of frequently communicating mail servers, especially major ISPs and partners.
Utilize tools like OpenSSL or online SSL checkers to verify your server's TLS certificate details and key size.
Implement DMARC reporting to gain visibility into TLS encryption rates for your outbound email traffic.
Expert view
Expert from Email Geeks says: The issue often arises when one server's certificate key size is smaller than the minimum required by the other server.
2017-05-18 - Email Geeks
Expert view
Expert from Email Geeks says: SSL/TLS key size errors pertain specifically to the encryption of the SMTP transaction, not DKIM authentication.
2017-05-18 - Email Geeks
Ensuring secure email communication
SSL/TLS key size errors in email SMTP transactions are a clear indicator that your email infrastructure's security posture might be out of sync with modern standards. Ensuring that your mail servers use robust TLS certificates with adequate key lengths is not just a technical formality, but a critical component of maintaining reliable email deliverability and protecting your sender reputation.
By proactively addressing these key size issues and staying updated on TLS best practices, you can prevent email delivery disruptions, avoid getting caught on a blacklist, and ensure your messages consistently reach their intended inboxes securely. Regular vigilance and configuration adjustments are key to seamless and protected email communication.
Microsoft 365 and 
Gmailstrictly enforce these security measures.
