http://www.rfc-editor.org/rfc/rfc2632.txt
RFC 2632: S/MIME Version 3 Certificate Handling, B. Ramsdell1999-06-01T23:00:00-00:00S/MIME (Secure/Multipurpose Internet Mail Extensions), provides a method to send and receive secure MIME messages. Before using a public key to provide security services, the S/MIME agent MUST certify that the public key is valid. S/MIME agents MUST use PKIX certificates to validate public keys as described in the Internet X.509 Public Key Infrastructure (PKIX) Certificate and CRL Profile. [STANDARDS-TRACK]