Why am I getting TLS errors when sending to Gmail?
Michael Ko
Co-founder & CEO, Suped
Published 29 Jul 2025
Updated 17 Aug 2025
9 min read
Suddenly encountering TLS errors when sending emails to Gmail recipients can be a frustrating experience. It often means your messages aren't reaching their destination, or they're being delayed significantly. I've seen many senders struggle with this, especially as email providers like Gmail tighten their security requirements.
The core of the issue usually revolves around the Transport Layer Security (TLS) protocol, which is essential for encrypting email communications. Without a proper TLS connection, Gmail might reject your emails, rate limit them, or flag them as suspicious. Understanding these errors is key to maintaining good email deliverability.
The role of TLS in email security
TLS, or Transport Layer Security, is the standard cryptographic protocol that ensures data privacy and integrity for communications over a computer network. In email, TLS encrypts the connection between your sending mail server and the recipient's mail server, preventing eavesdropping and tampering. It's the digital handshake that keeps your emails secure as they travel across the internet.
For email, the secure connection is typically established using STARTTLS, a command that upgrades an insecure connection to a secure one, or through implicit TLS on a dedicated port. Gmail has a strong preference, and increasingly a requirement, for all inbound and outbound mail to be secured with TLS. This commitment to security is why you might see specific error messages like 421-4.7.29 Your email has been rate limited because this message wasn't sent over a TLS connection. Gmail requires all bulk email senders to use TLS/SSL for SMTP connections.. This specific rate limit message highlights that if you're sending bulk email, TLS is non-negotiable.
Evolution of TLS requirements
Email security standards have been evolving, and mail providers are continuously raising the bar. What might have worked a few years ago could now result in delivery failures. For instance, many providers, including Google, have deprecated older TLS versions like TLS 1.0 and TLS 1.1 due to security vulnerabilities, favoring TLS 1.2 and newer. If your server is still attempting to use an outdated version, it will likely be rejected.
The stricter enforcement also means that even if your server supports TLS, any misconfiguration can lead to problems. You can use the Google Transparency Report to check if your domain's outgoing mail is using TLS and verify the current security status.
Diagnosing common TLS errors
When you encounter TLS errors, it's typically due to an issue with how your mail server (or your Email Service Provider's server) is handling the secure connection. One of the primary culprits is related to server certificates. A TLS connection relies on a valid SSL/TLS certificate to prove the server's identity and encrypt the communication.
If the certificate is expired, revoked, or not correctly configured, the TLS handshake will fail. Another common scenario is a mismatch between the hostname on the certificate and the hostname your mail server is using to connect. Gmail, being a large and vigilant receiver, will quickly identify these discrepancies and reject the connection.
Certificate related issues
An expired certificate is a straightforward problem but often overlooked. Like a driver's license, certificates have an expiration date, and once passed, they become invalid. A certificate can also be self-signed, meaning it's not issued by a trusted Certificate Authority (CA). While these can be used for internal purposes, they are generally not trusted by public email servers like Gmail, leading to authentication failures.
Another specific issue is when the mail server has multiple hostnames, and the one being used for the connection doesn't match the name on the server's certificate. This certificate doesn't match the host error can be tricky to diagnose, but it's a critical component of the TLS handshake. It's similar to SSL/TLS key size errors, where the certificate itself might be technically fine, but its parameters are not compatible with the receiving server's security policies.
TLS version and protocol mismatches
As mentioned earlier, the deprecation of older TLS versions plays a significant role. If your mail server only supports TLS 1.0 or 1.1, and Gmail (or any other major mailbox provider, like Outlook) has moved to TLS 1.2 or higher, the connection will fail. This can result in errors such as TLS Handshake Failed or SSL handshake failure.
Another related issue is the proper use of STARTTLS. Some servers might try to establish an unencrypted connection before issuing the STARTTLS command, or they might not offer it at all. If Gmail expects STARTTLS (which it generally does for port 587) and doesn't receive it, or if it encounters an error during the upgrade, the connection will be terminated, leading to a deferral or rejection. This aligns with why you might be seeing Google STARTTLS errors.
Troubleshooting and resolving TLS errors
When facing TLS errors, the first step is always to check your mail server's logs. These logs often provide the most direct insight into why the connection failed. Look for specific error codes or messages related to TLS, SSL, or certificate validation. If you're using an Email Service Provider (ESP), they should have tools or support to help you review these logs.
Next, use external tools to verify your TLS configuration from an outside perspective. Services that analyze your mail server's capabilities can confirm if your server is presenting a valid certificate, supporting the correct TLS versions, and properly handling STARTTLS. This can help isolate whether the issue lies with your server's setup or a specific interaction with Gmail.
Configuration fixes
The most common fixes involve updating your server's software, including your mail server application and underlying operating system components, to ensure support for modern TLS protocols like TLS 1.2 or TLS 1.3. For certificate issues, ensure your SSL/TLS certificate is current, valid, and properly installed. Crucially, the common name (CN) or a subject alternative name (SAN) on your certificate must match the hostname your mail server uses when connecting to other servers.
Always verify your SMTP port settings. Port 587 is standard for outgoing mail with STARTTLS, while port 465 is used for implicit TLS (SMTPS). Ensure your client and server configurations align with these expectations. If you are experiencing general email deliverability problems, you can review our guide on why your emails are going to spam and how to fix it.
TLS configuration tips
Certificate validity: Ensure your SSL/TLS certificate is not expired and is issued by a trusted Certificate Authority.
Hostname matching: Confirm the hostname used by your mail server matches the certificate's common name or a subject alternative name.
TLS version support: Verify your server supports TLS 1.2 or higher.
STARTTLS configuration: Ensure your server correctly initiates STARTTLS for outgoing connections.
Sometimes, the issue isn't a complete failure but a degradation in the TLS connection, which can still lead to errors or rate limiting. This is why it's important to differentiate between hard bounces and Gmail TempFail errors. A temporary failure often indicates a transient network issue or a momentary security check failure, whereas a persistent TLS error points to a fundamental configuration problem.
Common problems
Expired certificate: Mail server's SSL/TLS certificate has passed its expiration date.
Hostname mismatch: The name on the certificate doesn't match the hostname used for SMTP connection to Google.
Outdated TLS version: Server attempting to use TLS 1.0 or 1.1, which are often rejected.
STARTTLS negotiation failure: Server fails to initiate or complete the secure upgrade.
Solutions
Renew certificate: Obtain and install a new, valid SSL/TLS certificate.
Update hostname: Configure your mail server to use a hostname that matches the certificate.
Upgrade TLS support: Ensure your server's software supports and prefers TLS 1.2+.
Verify STARTTLS: Check server configuration for proper STARTTLS initiation on relevant ports.
It's also worth noting that an IP address being listed on a blacklist (or blocklist) can sometimes present with symptoms similar to TLS errors, as receiving servers might refuse connections altogether, or degrade them, without explicitly stating a blocklist issue. While not a direct TLS problem, it's part of the broader deliverability picture.
Ensuring secure email delivery
Email security is a continuously evolving field, and major providers like Gmail are at the forefront of implementing stricter requirements to protect users from spam and phishing. TLS is a fundamental layer of this security. Ignoring TLS errors can severely impact your email deliverability and sender reputation, leading to your legitimate emails being blocked or sent to the spam folder.
Proactive monitoring of your mail server's health, including its TLS configuration, is essential. Regularly check certificate expiration dates and stay updated on the latest security protocols. By ensuring your emails are sent over secure, properly configured TLS connections, you demonstrate reliability and trustworthiness, which are critical for reaching the inbox.
If you're still struggling after trying these steps, consider utilizing an email deliverability tester or consulting with an email security specialist. Investing in proper TLS setup and maintenance is an investment in your sender reputation and overall email program success.
Views from the trenches
Best practices
Always ensure your mail server's SSL/TLS certificate is current and hasn't expired to avoid connection rejections.
Verify that the hostname your server uses for connections matches the certificate's common name or a subject alternative name to prevent hostname mismatch errors.
Configure your mail server to support and prefer modern TLS versions, such as TLS 1.2 or TLS 1.3, as older versions are increasingly deprecated.
Common pitfalls
Using self-signed SSL/TLS certificates for public-facing email sending, as these are typically not trusted by major mail providers like Gmail.
Failing to update server software and libraries, leading to a lack of support for required TLS versions.
Not properly configuring STARTTLS, causing the secure connection upgrade to fail during the SMTP handshake.
Expert tips
Perform regular email deliverability tests to verify your TLS connection and identify any potential issues before they become critical.
If using an ESP, confirm their TLS configuration aligns with Gmail's latest requirements to ensure seamless delivery.
For complex setups, consider consulting a specialist to perform a thorough audit of your mail server's TLS implementation.
Expert view
Expert from Email Geeks says that Google often flags situations where STARTTLS is not being properly utilized by the sender's server.
2024-12-10 - Email Geeks
Expert view
Expert from Email Geeks says it's crucial to check server logs and traffic for any deferred transactions to understand why the TLS negotiation is failing.
Gmail tighten their security requirements.
Outlook) has moved to TLS 1.2 or higher, the connection will fail. This can result in errors such as TLS Handshake Failed or SSL handshake failure.
Google.
