Read information about Thunderbird's certificate store - it has a separate store, which may need to add the right certificate. What happens if you add a security exception, using the prompt in the security error message? Your anti-virus may also be interfering with Thunderbird's access to certificates. Check online for how to manually add a certificate for Kasperksy, Avast and other virus software. Virus scanners often conflict with email clients, so it's worth turning off the mail scan in the virus software, or make an exception to prevent scanning / blocking Thunderbird.

Also answered here: https://support.mozilla.org/en-US/questions/1352842

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