The Request for Comments (RFCs) listed below define MIME and Secure MIME (S/MIME). You can download the RFCs from the Internet Engineering Task Force (IETF) Web site.

RFC 1847: Security Multiparts for MIME: Multipart/Signed and Multipart/Encrypted
RFC 2045: MIME Part One: Format of Internet Message Bodies
RFC 2046: MIME Part Two: Media Types
RFC 2047: MIME Part Three: Message Header Extensions for Non-ASCII Text
RFC 2048: MIME Part Four: Registration Procedures
RFC 2049: MIME Part Five: Conformance Criteria and Examples
RFC 2183: Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field
RFC 2630: Cryptographic Message Syntax
RFC 2632: S/MIME Version 3 Certificate Handling
RFC 2633: S/MIME Version 3 Message Specification
RFC 2634: Enhanced Security Services for S/MIME