Software⏱️ 3 min read📅 2026-06-19

How to Fix: Telnet to Gmail SMTP "starttls auth" syntax error

Telnet issue with Gmail SMTP 'starttls auth' syntax error, how it works and why it's necessary.

Quick Answer: The error occurs because Telnet doesn't support STARTTLS. To fix, use a different method like SSL/TLS or a third-party tool that supports STARTTLS.

📋 Table of Contents

  1. 🛑 Root Causes of the Error
  2. 🚀 How to Resolve This Issue
  3. 💡 Conclusion

The error 'Syntax error' when attempting to use STARTTLS AUTH with Gmail's SMTP server is typically caused by an incorrect or missing command in the Telnet sequence. This issue affects users who are trying to send emails using a non-standard protocol, such as Telnet, and not through their email client.

It can be frustrating for users who have successfully sent emails using Telnet in the past but now encounter this error. In this guide, we will explore possible causes of this error and provide steps to resolve it.

🛑 Root Causes of the Error

  • The primary reason for this error is that Gmail's SMTP server requires STARTTLS AUTH before establishing a secure connection. The 'Syntax error' message indicates that the server expects the AUTH command to be preceded by the STARTTLS command, but it was not provided in the Telnet sequence.
  • An alternative reason could be that the user has not enabled STARTTLS AUTH in their Gmail account settings or has an incorrect configuration for this feature.

🚀 How to Resolve This Issue

Enabling STARTTLS AUTH using the Gmail account settings

  1. Step 1: Open Gmail and navigate to the 'Settings' page by clicking on the gear icon in the upper right corner. Select 'See all settings' from the dropdown menu.
  2. Step 2: In the 'General tab', scroll down to the 'Security' section and click on the link that says 'Start TLS'. Make sure the toggle switch is turned ON.
  3. Step 3: Click 'Save Changes' to apply the changes.

Using a third-party email client or application

  1. Step 1: Consider using a third-party email client or application that supports STARTTLS AUTH, such as Microsoft Outlook or Mozilla Thunderbird.
  2. Step 2: These clients often provide an easier and more secure way to send emails without the need for manual Telnet commands.

💡 Conclusion

To resolve the 'Syntax error' when attempting to use STARTTLS AUTH with Gmail's SMTP server, enable STARTTLS AUTH in your account settings or consider using a third-party email client. By following these steps, you should be able to send emails securely and without any errors.

Did this fix your problem?

If not, try searching for specific error codes.

🔍 Search Error Database

❓ Frequently Asked Questions

🛠️ Related Fixes

How to Fix: Pc crashes shortly after launching game (rainbow

Fix Pc crashes shortly after launching game (rainbow six siege). Compl

How to Fix: Installing an APK on a locked down phone

Installing an APK on a locked down phone: Try using a rooted device, e

How to Fix: New PC build- no signal and no clue

Fix New PC build- no signal and no clue. Complete troubleshooting guid