Thunderbird

Fix Thunderbird the Certificate is Not Valid for Server Error

By Admin

Updated: April 16, 2026 ~ 5 Minutes Reading

Table of Contents

“Thunderbird the certificate is not valid for that server” is a common error experienced with multiple account types, such as Gmail, Outlook, bellsouth.net, etc. This Thunderbird certificate error is caused by the Mozilla Thunderbird application not trusting the server’s security certificate, which leads to users not being able to send or receive email.

Here, we have explained what the symptoms, causes, fixes, and effects of Thunderbird not trusting certificate issues are.

What Does Thunderbird Certificate Error Mean?

Thunderbird, as an email application, fetches email from your email provider’s server by establishing a secure connection based on SSL or TLS certificates. When Thunderbird finds some fault in these certificates, it returns the “Thunderbird: the certificate is not valid for that server” error message on your screen. In most cases, it is a false alarm, and you can continue by adding a security exception.

Causes of Thunderbird the Certificate is Not Valid for That Server Error

Thunderbird does not recognize or trust the certificate due to the following reason:

  • Certificate is Self-signed – common if you are using an internal server.
  • Incorrect server settings – especially connection security or server mismatch.
  • Antivirus or Proxy – it modifies certificates with its own.
  • Due to corrupted cached certificates.
  • Expired Certificate – due to incorrect date & time or genuine expiration.

How to Fix Thunderbird Certificate Error on Windows & Mac?

The given troubleshooting steps apply to the latest Thunderbird versions on all devices.

Note: The error does not lead to any sort of email data loss. Even if you can not see the emails in Mozilla Thunderbird, they are safely stored on your email server. You can access them via Webmail or after you have resolved the issue.

Fix 1: Use the Add Security Exception Option

If you are using a self-signed certificate, or you are certain about the security of your email provider. You can add an exception to pass through the “Thunderbird: the certificate is not valid for that server.”

  1. Click on the Add Exception button when the Thunderbird certificate error pops up.
  2. Click on Get Certificate >> View Certificate.
  3. Confirm to set up a security exception.

If your Thunderbird continues to ask for certificate confirmation and will not make a permanent exception for any of them. In such cases, try out the given fixes below.

Fix 2: Thunderbird Certificate Manager to Remove Old Certificates

  1. Go to Menu (three lines) >> Settings.
  2. Open the Privacy & Security tab.
  3. Scroll to the certificate section.
  4. Click on Manage Certificates.
  5. Go to the Server tab.
  6. Delete any malformed server, severely no longer in use, or any random server you observed.
  7. Restart Thunderbird.

In most cases, old corrupted servers lead to the “Thunderbird: the certificate is not valid for that server” issue.

Note: You may need to reconfigure the account after removing distrusted old servers.

Fix 3: Verify Date & Time if Thunderbird Certificate Error Persists

An incorrect date & time can make Thunderbird consider the SSL or TLS certificate as invalid or expired in Windows 10/11.

  1. Open Settings on Windows.
  2. Go to Time & Language >> Date & Time.
  3. Toggle on Set Time Automatically.
  4. Restart your PC to fix the issue.

Fix 4: Check Server Hostname & Security Settings

  1. Open Mozilla Thunderbird on your computer.
  2. Click on Navigational Menu >> Account Setting.
  3. Open the Server Settings tab.
  4. Ensure connection security is set to “SSL/TLS.”
  5. Verify that the Port Name and Server Name are correct and there is no typo.

If you do not know the correct server settings for your email account, they are always available on the website of the email service provider.

Fix 5: Disable Interruptions

Turn off the antivirus, Proxy, and Firewall for email on your computer.

These interruptions can alter the server hostname, port number, and security settings, ultimately disrupting the connection.

Generally, the Thunderbird certificate error with Gmail, Outlook.com, etc., is fixed by trying all of the manual fixes mentioned here. However, if the issue continues, reach out to your email service provider to verify the conflict on the server end.

Not Able to Access Email due to Thunderbird Certificate Error?

Use Thunderbird Converter to export Thunderbird emails in MBOX, EML, PST, HTML, and many other formats to access emails with attachments and all other email properties without an active account, or even if Thunderbird crashes. Give the free trial version of the tool today.

FAQ: Thunderbird the Certificate is Not Valid for That Server

Q1: How to fix Thunderbird certificate error on Android?

If you get Thunderbird, which does not recognize the certificate on the Android application. You need to remove and re-add the account to fix the issue.

Q2: Where does Thunderbird store certificates?

You can access Thunderbird certificates via the security manager in Settings >> Privacy & Security. They are stored in cert9.db file in Thunderbird profile.

Q3: What certificate is Thunderbird referring to in the “Thunderbird not trusting certificate” issue?

SSL or TLS certificates, either self-signed or from a reputable company.