There is a very good guide on how to obtain and install Thawte mail certificates in Thunderbird.
http://www.heypete.com/pete/secure/certs/smime/thawte/firefox.html
But when you’ll try to sign or encrypt the mail with this certificate, Thunderbird will refuse to send the mail as the certificate is not verified.
Your mail certificate is signed by Thawte Personal Freemail Issuing CA certificate, so for verification Thunderbird requires this certificate.
Apparently (at least in my case) Thunderbird doesn’t come with this certificate. So to fix this problem we will have to manually install Thawte certificate.
- Download Thawte certificates from this page: https://www.thawte.com/roots/index.html and unzip the files
- Go to Thunderbird “Tools/Options/Advanced/Certificates” and click “View Certificates”
- Click on “Authorities” tab
- Click Import
- Browse to the folder with Thawte certificates and choose “Thawte Personal Root Certificates\Thawte Personal Freemail Issuing CA\ Thawte Personal Freemail Issuing CA.cer” (Just in case I also installed “Thawte Personal Root Certificates\Thawte Personal Freemail CA\ Thawte Personal Freemail CA.cer”)
Now when you click your certificate it should say that the certificate has been verified and you can send your encrypted or signed e-mails
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Feel free ask any questions or leave comments if it was helpful.