S/MIME and PGP/MIME Fully outlines the details of the S/MIME and PGP/MIME protocols. In addition, this page offers a current status and related documents.
Secure E-mail Standard TechWeb news article (November 1997) on S/MIME becoming a formal standard via the Internet Engineering Task Force (IETF).
IETF's S/MIME Working Group Describes the IETF S/MIME working group and includes links to Internet draft documents for S/MIME 2 and 3 as well as other Internet drafts that related to this work.