I'm maintaining a WordPress Page for a client that now wants E-Mails to be signed off using S/MIME. Currently we're using the WP Mail SMTP Client Plugin to send E-Mails. They don't offer S/MIME. Has anyone successfully tried using S/MIME and is there maybe a way to hook into an action and add the encryption that way? I'm not sure how to implement the task for WordPress.
So far I've tried googling the issue, but it doesn't seem like anyone has ever done this. There are some very outdated Plugins, but nothing up-to-date.