S-MIME

See also:

Secure MIME (S/MIME) - Comparison between OpenPGP and S/MIME, plus the RFC's that relate to the Secure MIME standard.
S/MIME Freeware Library (SFL) - An open source implementation of the IETF S/MIME v3 RFC 3369 Cryptographic Message Syntax (CMS) and RFC 2634 Enhanced Security Services (ESS) specifications.
S/MIME Working Group - The resource site of IETF S/MIME working group, with links to draft specifications. (Internet Mail Consortium)
SMIME.org - Promotes email encryption usage, by providing help and references to email encryption products and standards.