Secure/Multipurpose Internet Mail Extensions (S/MIME)

A set of specifications for securing electronic mail. S/MIME is based upon the widely used MIME standard [MIME] and describes a protocol for adding cryptographic security services through MIME encapsulation of digitally signed and encrypted objects. The basic security services offered by S/MIME are authentication, nonrepudiation of origin, message integrity, and message privacy. Optional security services include signed receipts, security labels, secure mailing lists, and an extended method of identifying the signer’s certificate(s).
SOURCE: SP 800-49; CNSSI-4009