Why am I getting TLS errors when sending to Gmail?

Summary

TLS errors when sending to Gmail are multifaceted, stemming from issues on both the sender's and receiver's ends. Core issues involve TLS configuration (ensuring TLS is enabled and correctly set up), certificate problems (expired, invalid, or untrusted certificates), and protocol mismatches (incompatible TLS versions or cipher suites). Gmail mandates TLS/SSL for bulk senders, and errors like `421-4.7.29` indicate a failure to send via a TLS connection. Additionally, problems can arise from a lack of STARTTLS support, which enables encryption after a plain text connection. Gmail may also enforce rate limiting or filter for spam, triggering temporary errors. Authentication failures (SPF, DKIM, DMARC) and outdated SSL libraries further complicate matters. Experts recommend validating TLS setups (using tools like aboutmy.email or MXToolbox), ensuring certificate validity, reviewing protocol compatibility, configuring STARTTLS, and analyzing mail server logs. Possible reasons for errors on GSuite SMTP include Google rate limiting, detecting possible SPAM, or lack of user authentication.

Key findings

  • TLS Configuration Issues: Improper TLS configuration, including missing or incorrect settings, is a primary cause.
  • Certificate Problems: Invalid, expired, or untrusted SSL/TLS certificates lead to connection failures.
  • Protocol Mismatches: Incompatible TLS versions or cipher suites prevent successful handshakes.
  • STARTTLS Support: Lack of STARTTLS support hinders secure connection negotiation.
  • Gmail Enforcement: Gmail requires TLS/SSL for bulk senders and may enforce rate limits or spam filtering.
  • GSuite SMTP Issues: Google may rate limit accounts, detect possible SPAM, or the user needs to authenticate with Google

Key considerations

  • Verify TLS Configuration: Confirm TLS is enabled, properly configured, and supports Gmail's requirements.
  • Ensure Certificate Validity: Validate the SSL/TLS certificate, install it correctly, and ensure it's trusted.
  • Check Protocol Compatibility: Review supported TLS versions and cipher suites for compatibility with Gmail.
  • Implement STARTTLS: Configure and enable STARTTLS on the SMTP server.
  • Monitor Rate Limits: Stay within Gmail's sending limits and avoid spam-like behavior.
  • Authenticate Emails: Set up SPF, DKIM, and DMARC to improve email authentication and trustworthiness.
  • GSuite Account Considerations: Ensure GSuite account authentication is configured correctly and monitor sending for rate limiting

What email marketers say
10Marketer opinions

TLS errors when sending to Gmail can stem from various issues related to TLS configuration, compatibility, and authentication. Common causes include TLS handshake failures due to mismatched protocol versions or cipher suites, incorrect or expired SSL certificates, SMTP servers not configured for STARTTLS, and network or firewall issues. Gmail may also enforce rate limiting or detect potential spam, leading to TLS-related errors. Ensuring proper email authentication (SPF, DKIM, DMARC) and trusted SSL/TLS certificates are critical for resolving these issues.

Key opinions

  • Handshake Failures: Mismatched TLS protocol versions or cipher suites can cause TLS handshake failures.
  • STARTTLS Configuration: SMTP servers not properly configured to use STARTTLS can lead to TLS errors.
  • Certificate Issues: Expired, invalid, or untrusted SSL certificates are common causes of TLS errors.
  • Authentication Problems: Improperly configured SPF, DKIM, and DMARC records can contribute to TLS errors.
  • Gmail Rate Limiting: Gmail may rate limit accounts or detect spam, leading to temporary TLS-related issues.

Key considerations

  • TLS Version Support: Ensure the email server supports the TLS version required by Gmail.
  • SSL Certificate Validity: Verify the SSL certificate is valid, properly installed, and trusted by Gmail.
  • STARTTLS Verification: Confirm the SMTP server is configured to use STARTTLS.
  • Email Authentication Setup: Properly configure SPF, DKIM, and DMARC records to authenticate emails.
  • Rate Limiting Checks: Monitor email sending volumes to avoid exceeding Gmail's rate limits and triggering errors.
Marketer view

Email marketer from Reddit shares possible reasons for a GSuite SMTP 421 error, including Google rate limiting accounts for sending too much email, Google detecting possible SPAM (so is limiting accounts), or the user needs to authenticate with Google

June 2024 - Reddit
Marketer view

Email marketer from MXToolbox shares that using the MXToolbox SMTP Test tool can help diagnose TLS connection issues by testing if the server supports STARTTLS and what TLS versions are available.

February 2022 - MXToolbox
Marketer view

Email marketer from DigitalOcean shares that If using Let's Encrypt, ensure the certificate is automatically renewed to prevent expiration. Errors can arise if the renewal process fails.

July 2024 - DigitalOcean
Marketer view

Email marketer from StackOverflow shares that TLS handshake failures can occur if the client and server don't support a common TLS protocol version or cipher suite. Also firewall or network issues can interrupt the TLS handshake.

November 2024 - StackOverflow
Marketer view

Email marketer from cPanel Forum explains that TLS errors can occur if the SSL certificate is not correctly installed or has expired. Ensure the certificate is valid, properly installed for the SMTP service, and trusted by the receiving server.

February 2025 - cPanel Forum
Marketer view

Email marketer from Reddit responds by suggesting checking the email server's TLS configuration and ensuring it supports the required TLS version. They also mention it could be a temporary issue on Gmail's side.

November 2022 - Reddit
Marketer view

Email marketer from StackExchange responds by suggesting that issues with SSL/TLS could stem from outdated SSL libraries or configurations on the server, particularly if the server's cipher suites are not compatible with Gmail's requirements. Update SSL libraries, review supported cipher suites, and confirm your SSL certificate is correctly installed.

November 2022 - StackExchange
Marketer view

Email marketer from EasyDMARC shares that ensuring proper authentication configurations with SPF, DKIM, and DMARC can reduce TLS errors. Proper authentication allows sending servers to be validated, which can improve the connection.

October 2021 - EasyDMARC
Marketer view

Email marketer from SuperUser suggests verifying that the SMTP server is configured to use STARTTLS. They also suggest checking the mail server logs for specific error messages.

September 2021 - SuperUser
Marketer view

Email marketer from AuthSMTP shares that often the issue is the SSL/TLS certificates that you or your recipient are using are not trusted, either because they are self-signed, or signed by an untrusted Certificate Authority. Your mail client or server must trust the Certificate Authority that signed the SSL/TLS certificate for the connection to work

May 2023 - AuthSMTP

What the experts say
7Expert opinions

TLS errors when sending to Gmail are often attributed to issues with the sender's TLS configuration. These include problems like incorrectly configured TLS settings, expired or invalid SSL certificates, and mismatched TLS protocol versions. Gmail may also enforce TLS requirements, or issues could be related to sending volume or sender reputation. Experts recommend verifying TLS setups using tools like aboutmy.email, checking for certificate validity, ensuring proper TLS configuration, and analyzing mail server logs for systemic problems. They also suggest that Gmail thinks STARTTLS isn't being used.

Key opinions

  • TLS Configuration: Incorrectly configured TLS settings on the sender's side is a primary cause of TLS errors.
  • Certificate Issues: Expired or invalid SSL certificates are common culprits behind TLS errors.
  • Protocol Mismatches: Mismatched TLS protocol versions between the sender and Gmail can lead to errors.
  • Gmail Enforcement: Gmail may enforce TLS requirements, causing errors if the sender doesn't comply.
  • STARTTLS Usage: Google may think STARTTLS isn't being used.

Key considerations

  • Verify TLS Setup: Use tools to verify the TLS setup is correct and functional.
  • Check Certificate Validity: Ensure the SSL certificate is valid and properly installed.
  • Review Protocol Versions: Check for compatibility issues with Gmail's required TLS protocol versions.
  • Analyze Logs: Analyze mail server logs for systemic issues, particularly if throttling is frequent.
  • Consider Gmail Requirements: Be aware of and adhere to Gmail's current TLS requirements and best practices.
Expert view

Expert from Email Geeks explains the need to configure or fix TLS on outbound emails, suggesting the use of aboutmy.email to diagnose the issue.

August 2022 - Email Geeks
Expert view

Expert from Email Geeks suggests that Google thinks STARTTLS is not being used and recommends analyzing logs for systemic issues if throttling is frequent.

November 2022 - Email Geeks
Expert view

Expert from Word to the Wise shares that TLS errors can occur if the email server is not configured to use TLS, the TLS certificate has expired, or the TLS version is not supported by the recipient's server. It is important to ensure that TLS is enabled, that the TLS certificate is valid, and that the TLS version is supported by both sender and receiver. The TLS configuration can be checked using online tools such as SSL Checker or by inspecting mail server logs.

July 2024 - Word to the Wise
Expert view

Expert from Email Geeks recommends using aboutmy.email to examine TLS setup details for better advice, downplaying the likelihood of a Gmail error.

April 2023 - Email Geeks
Expert view

Expert from Email Geeks suggests checking the TLS setup to ensure it's working correctly, indicating a possible issue on the sender's side rather than Gmail's.

March 2023 - Email Geeks
Expert view

Expert from SpamResource shares that TLS errors arise due to certificate issues, mismatched protocols, or problems negotiating encryption. Start by verifying your SSL/TLS configuration, ensuring the certificate is valid and properly installed, and checking for compatibility issues with Gmail's required protocols. If problems persist, check if recent software changes might have affected encryption and run diagnostics to verify TLS support.

February 2023 - SpamResource
Expert view

Expert from Email Geeks says that Gmail may have started enforcing TLS, or it could be related to volume or reputation and indicates that the certificate might have expired.

September 2023 - Email Geeks

What the documentation says
3Technical articles

TLS errors when sending to Gmail often stem from the requirement that all bulk email senders use TLS/SSL for SMTP connections, as indicated by the `421-4.7.29` error. These errors can arise from configuration mismatches, certificate problems, or protocol incompatibilities. STARTTLS, which allows SMTP servers to negotiate TLS encryption after a plain text connection is established, is critical, but requires support from both the client and server.

Key findings

  • Gmail Requirement: Gmail requires all bulk email senders to use TLS/SSL for SMTP connections.
  • Error Indication: The `421-4.7.29` error signals that the message wasn't sent over a TLS connection.
  • Common Causes: Configuration mismatches, certificate problems, or protocol incompatibilities are frequent causes of TLS errors.
  • STARTTLS Importance: STARTTLS is crucial for negotiating TLS encryption after a plain text connection.
  • Mutual Support: Both the client and server must support the STARTTLS extension for it to function correctly.

Key considerations

  • TLS/SSL Compliance: Ensure compliance with Gmail's TLS/SSL requirements for SMTP connections.
  • Configuration Review: Review TLS configurations to identify and resolve mismatches or incompatibilities.
  • Certificate Management: Address and resolve any certificate-related issues.
  • STARTTLS Implementation: Verify that STARTTLS is properly implemented and supported by both the client and server.
  • Connection Testing: Use tools like s_client to test the connection and diagnose issues.
Technical article

Documentation from Google Workspace Admin Help explains that Gmail requires all bulk email senders to use TLS/SSL for SMTP connections. The error `421-4.7.29` indicates that the message wasn't sent over a TLS connection.

April 2024 - Google Workspace Admin Help
Technical article

Documentation from RFC Editor specifies that STARTTLS allows SMTP servers to negotiate TLS encryption after establishing a plain text connection. It's important that both the client and server support this extension.

September 2023 - RFC Editor
Technical article

Documentation from OpenSSL explains that TLS errors often arise from configuration mismatches, certificate problems, or protocol incompatibilities. The documentation recommends using s_client to test the connection and diagnose the specific issue.

March 2024 - OpenSSL