S/MIME (Secure/Multipurpose Internet Mail Extensions) is a protocol that adds digital signatures and encryption to Internet MIME (Multipurpose Internet Mail Extensions) messages.
MIME is the standard for Internet mail that makes it possible to send more than text. A mail message is splits into two parts, the header, which contains the information needed to move the mail from the source to its destination and the body. The MIME structure allows an e-mail body to contain graphics, audio and many other features that improve communication over simple text. Almost all modern e-mail systems support it.
S/MIME added two kinds of security to e-mail. It added the ability to encrypt e-mail so that only the intended receiver could read it. It also added the ability to authenticate the author of an e-mail message. Both of these security measures depend upon asymmetric encryption, public-key technologies.
Since these activities require an appropriate public-key infrastructure, and must work in every popular mail package in order to be useful; an open, standards-based approach was required. RSA Security published the S/MIME standard, and some of the underlying technologies in 1997; it is now owned by an IETF (Internet Engineering Task Force) working group.