MacOS 10.13.6 Thunderbird: 91.6.1 Somebody is trying to send me an e-amil which includes an attachment. This attachment isn't shown in Thunderbird. I can see it in the e-mails source code though. If I log into my e-mail account via Firefox, I can see the attachments and download them. Usually every attachment works, but this doesn't. There is a small red button with the text "Bad Signature" visible when I open the e-mail and it lets me know that the digital signature isn't valid which probably is the reason why I can't view or download the attachment. The little info box that opens when I click the bad signature button shows me this: Signed by: Secure Mail: SEPPmail Certificate Email address: (redacted e-mail address for privacy reasons) Certificate issued by: SwissSign RSA SMIME LCP ICA 2021 - 2 How can I set up Thunderbird to accept this certificate? I tried downloading it via swisssign.com but can't import the files I can download on their site in Thunderbird. Files have the .ca ending and are flagged as PEM or DER. Anything I can do on my end or is this a configuration error on the senders end? Thanks.