Here you can select the certificate to S/MIME-encrypt messages when this identity and when encrypting emails, always also encrypt to the certificate of my own identity are in effect.
To change the selected certificate, press the Change... button. A dialog listing all S/MIME encryption certificates found in your local keybox will be shown allowing you to select the one to use.
You can clear the list of certificates and get more information about them in the same way as described for OpenPGP signing key.
