S/MIME (Secure/Multipurpose Internet Mail Extensions) is a standard for signing and encrypting e-mails end-to-end. Usually signing, verifying and encrypting/decrypting is done by the mail client.
Using S/MIME with your standalone email client such as Thunderbird, Outlook or Apple Mail is supported.
Zone does not currently provide built-in support of S/MIME in Webmail.
If S/MIME support in Webmail is something you’d like to see, let us know.