Hi All. New to Thunderbird but liking what I see. Am trying to install a new certificate but keep seeing the same error message when I attempt to send an email: Sending of the message failed. You specified that this message should be digitally signed, but the application either failed to find the signing certificate specified in your Mail & Newsgroup Account Settings, or the certificate has expired. So here is what we know: The certificate seems to work (I've sent test emails to a couple of test email accounts) when I use it in Outlook safe mode. Can't get it to work in normal Outlook mode because of add-ins, hence my switch to Thunderbird. They seem to appear fine in the S/MIME section for digital signing and encryption and I can even see the certificate when I go into Manage S/MIME Certificates. Has anyone come across this before / got any ideas about how to resolve this? Thanks!!