Secure Multipurpose Internet Mail Extensions

Secure/Multipurpose Internet Mail Extensions (or S/MIME) is a protocol for sending messages that are encrypted and digitally signed. S/MIME provides for cryptographic security services such as authentication, message integrity, and non-repudiation of origin (using digital signatures).

History

The first version of S/MIME was developed in 1995 by a number of security vendors. It was one of several specifications for message security.  Other distinct security applications included Pretty Good Privacy.  Over the following five years, S/MIME was adopted as the standard for message security. 

Functionality

S/MIME requires the use of a dedicated email client, and the installation of a security key, or certificate from a Certificate Authority (CA).