Install claws-mail-smime-plugin by entering the following commands in the terminal:
sudo apt update sudo apt install claws-mail-smime-plugin
S/MIME signature/encryption handling for Claws Mail
This plugin handles S/MIME signed and/or encrypted mails in Claws Mail. You can decrypt mails, verify signatures or sign and encrypt your own mails. . S/MIME is also known as PKCS#7 and uses the application/pkcs7-signature or application/x-pkcs7-signature MIME subtype within messages. . Receiving an attachment called smime.p7m is also sign of this kind of signature usage.