Implemented a SMIME-milter into postfix and tried to sign/encrypt emails using the Let's Encrypt certificate.
It successfully sign the email, but other providers/email-clients don't trust.
The certificate used to sign the message was issued by a certificate authority that you do not trust for issuing this kind of certificate.
Das Zertifikat, das zum Erstellen dieser Signatur verwendet wurde, hat nicht die richtigen Eigenschaften, um für das digitale Signieren von Nachrichten verwendet zu werden.
The certificate that was used to create this signature does not have the right properties to be used for digitally signing messages.
Damn, it would be a great step forward to use these certificates for signing emails as well.