Thunderbird doesn't trust a valid CA Certificate

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, but it says the Digital Signature is invalid? Why is it invalid? I got an error when I try to reply back with a digitally signed and encrypted email: Sending of the message failed. You specified encryption for this message, but the application failed to find an encryption certificate for johndoe@gmail.com What I have tried so far: Exported the .crt then import it on "Manage Certificates" People's Tab But still got an error when trying to reply to the email