S/MIME (Secure / Multipurpose Internet Mail Extensions) is a protocol that adds digital signatures and encryption to Internet MIME (Multipurpose Internet Mail Extensions) messages described in RFC 1521. MIME is the official proposed standard format for extended Internet electronic mail. Internet e-mail messages consist of two parts, the header and the body. The header forms a collection of field/value pairs structured to provide information essential for the transmission of the message. The structure of these headers can be found in RFC 822. The body is normally unstructured unless the e-mail is in MIME format. MIME defines how the body of an e-mail message is structured. The MIME format permits e-mail to include enhanced text, graphics, audio, and more in a standardized manner via MIME-compliant mail systems. However, MIME itself does not provide any security services. The purpose of S/MIME is to define such services, following the syntax given in PKCS #7 (see Question 5.3.3) for digital signatures and encryption. The MIME body section carries a PKCS #7 message, which itself is the result of cryptographic processing on other MIME body sections. S/MIME standardization has transitioned into IETF, and a set of documents describing S/MIME version 3 have been published there.
S/MIME has been endorsed by a number of leading networking and messaging vendors, including ConnectSoft, Frontier, FTP Software, Qualcomm, Microsoft, Lotus, Wollongong, Banyan, NCD, SecureWare, VeriSign, Netscape, and Novell. Information on MIME can be found at ftp://ftp.isi.edu/in-notes/rfc1521.txt.