S/MIME (for Secure MIME, or Secure Multipurpose Mail Extension) is a security process used for e-mail exchanges that makes it possible to guarantee the confidentiality and non-repudiation of electronic messages. S/MIME is based on the MIME standard, the goal of which is to let users attach files other than ASCII text files to electronic messages. The MIME standard therefore makes it possible to attach all types of files to e-mails.
S/MIME was originally developed by the company RSA Data Security. Ratified in July 1999 by the IETF, S/MIME has become a standard, whose specifications are contained in RFCs 2630 to 2633. How S/MIME works
The S/MIME standard is based on the principle of public-key encryption. S/MIME therefore makes it possible to encrypt the content of messages but does not encrypt the communication.
The various sections of an electronic message, encoded according to the MIME standard, are each encrypted using a session key.
The session key is inserted in each section’s header, and is encrypted using the recipient’s public key. Only the recipient can open the message’s body, using his private key, which guarantees the confidentiality and integrity of the received message.
In addition, the message’s signature is encrypted with the sender’s private key. Anyone intercepting the communication can read the content of the message’s signature, but this ensures the recipient of the sender’s identity, since only the sender is capable of encrypting a message (with his private key) that can be decrypted with his public key.
Reference(s) used for this question: http://en.kioskea.net/contents/139-cryptography-s-mime RFC 2630: Cryptographic Message Syntax; OPPLIGER, Rolf, Secure Messaging with PGP and S/MIME, 2000, Artech House; HARRIS, Shon, All-In-One CISSP Certification Exam Guide, 2001, McGraw-Hill/Osborne, page 570; SMITH, Richard E., Internet Cryptography, 1997, Addison-Wesley Pub Co.