S/MIME (Secure Multi-Purpose Internet Mail Extensions) is a secure method of sending e-mail that uses the Rivest-Shamir-Adleman encryption system. S/MIME is included in the latest versions of the Web browsers from Microsoft and Netscape and has also been endorsed by other vendors that make messaging products. RSA has proposed S/MIME as a standard to the Internet Engineering Task Force (IETF). An alternative to S/MIME is PGP/MIME, which has also been proposed as a standard.
MIME itself, described in the IETF standard called Request for Comments 1521, spells out how an electronic message will be organized. S/MIME describes how encryption information and a digital certificate can be included as part of the message body. S/MIME follows the syntax provided in the Public-Key Cryptography Standard format #7.