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 firstname.lastname@example.org