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 Authentication failures (SPF, DKIM, DMARC) and outdated SSL libraries further complicate matters.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
10 marketer 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
2 Mar 2022 - 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.
13 Apr 2025 - MXToolbox
What the experts say
7 expert 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.
2 Jun 2024 - 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.
1 Jan 2024 - Email Geeks
What the documentation says
3 technical 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.
20 Mar 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.
13 Dec 2024 - RFC Editor
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