So Basically I sent a digitally signed email using a certificate issued by Sectigo RSA Client Authentication and Secure Email CA.

Now the recipient of the email replied to me with a certificate issued by COMODO RSA Client Authentication and Secure Email CA. It also said that the Digital Signature is not valid.

I tried replying with an encrypted message and digitally signed but it gives me an error:

So what I have done is exported the .crt file of the senders certificate and I added it into the "People's Certificate" Tab in the "Manage Certificate" settings but still it won't send the email

Sending of the message failed.
You specified encryption for this message, but the application failed to      
find an encryption certificate for [email protected]
  • Doessn't Thunderbird give you any more details about why the signature is considered invalid?
    – not2savvy
    Commented Feb 15, 2019 at 9:02
  • I think COMODO S/MIME certs may not be trusted anymore. Look at why the signature is untrusted, and also at whether the certificate is trusted (both valid and has a valid chain of trust to a trusted root authority) or not. If it isn't valid, Thunderbird will certainly not use it. If it doesn't chain to a trusted root, you will have to manually mark it as trusted (or import ant trust its root) if you consider it legit.
    – CBHacking
    Commented Feb 15, 2019 at 10:10


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