S/MIME (Secure/Multipurpose Internet Mail Extensions) is a standard for public key encryption and signing of MIME data. S/MIME is on an IETF standards track and defined in a number of documents, most importantly RFC 3369, 3370, 3850 and 3851.
It was originally developed by RSA Data Security and the original specification used the IETF MIME specification with the de facto industry standard PKCS#7 secure message format.
Change control to S/MIME has since been vested in the IETF and the specification is now layered on Cryptographic Message Syntax, an IETF specification that is identical in most respects with PKCS #7.
S/MIME functionality is built into the majority of modern email software and interoperates between them. Since it is build on CMS, MIME can also hold an advanced electronic signature.