A protocol defined in IETF RFCs 3850 through 3852 and 2634 for encrypting messages and creating certificates using public key cryptography. S/MIME is supported by default installations of many popular mail clients. S/MIME uses a classic, hierarchical design based on certificate authorities for its key management, making it suitable for medium to large implementations.
NIST SP 800-45 Version 2
A set of specifications for securing electronic mail. S/MIME is based upon the widely used MIME standard 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, non-repudiation 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).
CNSSI 4009-2015 under secure/ multipurpose internet mail extensions (S/MIME)
(NIST SP 800-49)