What interface design? The only thing that should be visible to most people is some sort of alert that explains that an email was blocked because the signature

What interface design? The only thing that should be visible to most people is some sort of alert that explains that an email was blocked because the signature was missing or invalid. Everything else should be setup by IT. The MUA should find (and verify) the key in the directory service and automatically handle the signing/signature-verification. You might need UI to use signatures with external email, but that is a separate problem.