S/MIME, or Secure/Multipurpose Internet Mail Extensions is the de facto standard for encrypting and signing mail. You can encrypt mail to keep prying eyes off of it. Signing though, is much more common as it addresses the issue of non-repudiation in many organizations, or giving people a way to make sure that the email that they think you sent really came from you. It was also available in GPG plug-ins for mail, back in the day. But S/MIME used to really be for people who thought the government was out to get them, work for government agencies, just liked to be kinda’ nerdy or actually had something to hide. But…