In compose view the plugin shows a little lock icon.
When lock clicked it opens up a password input and displays a select box. User enters a password and select some value. Then this 3 values are stored in the "X-*" headers of the composing mail.
Plugin (or a separate) app installs personal SSL certificates. SSL certificates are delivered to clients in the same zip.
SSL certificates (PKCS12) are used to communicate with our mail server.