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.