Step 1: Open Thunderbird and call up the settings
If the menu bar is not displayed, press the "Alt key". Then call up the entry Options from the menu item Settings.
Step 2: Call up the certificate menu
Step 3: Import the user certificate
On the Certificate Manager screen click "Your certificates" to import your personal user certificate.
Please check that the e-mail address of your Thunderbird account matches the one you specified in the certificate request. The upper- and lower case letters must also match the information in the certificate.
Step 5: Activate and assign your user certificate
Now select the sub-item "S/MIME Security".
In the section "Digital signing", over the button "Select", choose your user certificate and Thunderbird will automatically enable it.
Your user certificate is now successfully deposited in Thunderbird. We recommend checking the box "Digitally sign messages (by default)" so that all of your outgoing mail will be automatically cryptographically signed.
Confirm and close the settings with "OK".
Step 6: Encryption when composing an e-mail
With the menu item "S / MIME", you now have the ability to encrypt the emails that you compose. Please note, however, that you can only encrypt e-mails if you have already received a signed e-mail from all recipients of the e-mail you want to encrypt.