S/MIME signed mail - Unable to import certificate

Hello all, I bought a SMIME certificate from Certum. Signing mails with it in emclient works well. The recipient (also on emclient) sees that the email is successfully signed. But… the recipient cannot import the certificate in order to encrypt their reply to me. There is no “import certificate” button next to the text that says the mail is signed. I have also not found any other way to detach or somehow discover the certificate.

I checked settings, both of us have “Include keys” activated in the Signing/Encryption policy settings for the account (as well as globally). So… now I wonder… is emClient wrongfully or not at all attaching the keys?? I received an encrypted and signed message from the provider and that email clearly offered to import their keys. So… who’s at fault here? Where do I begin?

Any help?
Thanks.