Are you running some sort of anti-virus software?

Nope. I do use a VPN, but I have been using it for years. This message just started appearing.

what happens when you click get certificate. Can you view it?

I just tried. Nothing happened. Well I can see that there is a reaction that it indicates it was clicked (not like a dead link) but nothing else is shown. The box just remains there.

I've also received a notice that an update to the OS is available. Maybe I'll do that and see what happens.

Contact Your System Administrator

You mentioned that updates are disabled by your system administrator. This is the most likely cause of your problem.

Thunderbird and other browsers constantly get security updates to recognize new, trusted certificates.

Your version of Thunderbird may be too old to recognize the most recent security certificate that Google is using.

The best solution is to contact the person who manages your computer (your system administrator) and ask them to update Thunderbird to the latest version. This will fix the problem and keep you secure.

I do use a VPN, but I have been using it for years. This message just started appearing.

Does it work without the VPN?

Contact Your System Administrator You mentioned that updates are disabled by your system administrator. This is the most likely cause of your problem. Thunderbird and other browsers constantly get security updates to recognize new, trusted certificates. Your version of Thunderbird may be too old to recognize the most recent security certificate that Google is using. The best solution is to contact the person who manages your computer (your system administrator) and ask them to update Thunderbird to the latest version. This will fix the problem and keep you secure.

Thanks for the reply.

I'm the system administrator. Not sure why it says I disabled updates but I regularly get a whole host of updates which don't get installed until I click on the update and allow the installation. But I have the latest version of Thunderbird as listed on the website, released on August 19, 2025. So I don't think that's it.

Many antivirus and security suites include a feature called SSL/TLS inspection or Secure Connection Scanning. Since this certificate is from your security software and not from Gmail, Thunderbird correctly flags it as untrusted.

1. Check the settings of your security suite.

Look for a feature with names like "Enable SSL scanning," "Secure Connection Scanning," or "Web Shield."

Disable this feature for Thunderbird only or add an explicit exception for Thunderbird in your antivirus settings.

2. System-Level Certificate Store Issues

The problem may not be with Thunderbird's certificate store but with your operating system's, may not have the root certificates required to validate Google's certificate chain.

You can use a command-line tool to check the certificate being served by Gmail's server. This will show you exactly what Thunderbird is seeing.

On a Linux system, open a terminal and run the following command to see the certificate chain:

openssl s_client -connect imap.gmail.com:993 -showcerts

Examine the output to see if the full certificate chain is present and if any of the certificates are flagged as expired or invalid.

Thunderbird for Linux Mint mint-001 - 1.0 Updates disabled by system administrator

My best guess would be you're using the Thunderbird package provided by your distribution. Then the 'Updates disabled' notification would be the expected behavior.

You mentioned that updates are disabled by your system administrator. This is the most likely cause of your problem.

I'd argue it isn't.

Many antivirus and security suites include a feature called SSL/TLS inspection or Secure Connection Scanning.

The OP is running Linux, and already stated they are not running an anti-virus software.

Since this certificate is from your security software and not from Gmail, Thunderbird correctly flags it as untrusted.

How do you determine the cert is from a security software the OP is (not) running?