Set up Thunderbird for S/MIME


During the last couple of days I tried to set up my mail clients to sign mail with S/MIME using a certificate that I got for free from StartCom. This includes a mail client on my Android phone, on my laptop and desktops. I already use the certificate on my domains and I added my email address a couple of months ago. So everything was prepared already. So this post is about the set up of the clients only.

On my Android phone I switched from K9 Mail to MailDroid, which is a fairly expensive app. Currently I’m still using the free version but I’m planning to pay the 20€ and get rid of the ads soon. To enable S/MIME signing and encryption, you have to install the FlipdogSolutions Crypto Plugin as well. After starting the crypto app, you can import your certificate and that’s it for signing and encrypting your mail on Android.

On the desktops and laptops it was a bit more hassle to get set up. I am using Thunderbird as my mail client and tried to follow the official documentation on S/MIME. But when I tried to send a signed mail with my certificate, I would get an error dialog complaining about missing trust for the certificate. After researching articles on S/MIME (all several years old, looks like everyone gave up the encryption stuff for private email) I decided to just re-import all root certificates from StartCom and I discovered that I missed the StartCom Class 1 Primary Intermediate Client CA in Thunderbird.

To fix this error, you can go to the StartSSL Control Panel, click on the menu StartCom CA Certificates in your Tool Box and download the StartCom Class 1 Primary Intermediate Client CA certificate. Import it into Thunderbird as described in the docs and you should be able to send signed and encrypted mail without any issues.

Enjoy your new privacy! :smile:

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