Dear Thunderbird Support Team, I'm experiencing an issue with using my digital signing certificate in Thunderbird. I have imported a p12 certificate into Thunderbird on my Ubuntu system, and everything seemed to go fine during the import. However, when I try to digitally sign my emails, I receive an error message that says, "Certificate Manager can't locate a valid certificate that can be used to digitally sign your messages with an address of [XXX@XYZ]." It is hosted by an academic institution using office365 products. I have checked the certificate details and noticed that the Basic Constraints section shows "Certificate Authority: No" marked as critical. I’ve seen that certificates on the internet often have different settings, and I’m wondering if this might be causing Thunderbird to not be able to find my certificate. I have checked the authority settings and both boxes are marked. I am running the latest version of Thunderbird on Ubuntu, and I have verified that the certificate is imported under "Your Certificates" in Thunderbird's Certificate Manager. My email address in Thunderbird matches the one on the certificate, but the error persists. I would appreciate any help. Best regards, Thomas G.