S-MIME is a specification under rfc2311 & rfc 2312 for secure electronic mail.

It stands for Secure/Multipurpose Internet Mail Extensions and was designed to add security to e-mail messages in MIME format. The security services offered are authentication through digital signatures and privacy using encryption. S/MIME is capable of hiding the PKI certificate and and making the sender anonymous during transit through encrypted data encapsulation.

When planning to implement S-MIME or S/MIME, be sure the software package has achieved the "S/MIME-Enabled" seal. Some software that is currently compatable:

Entrust Technologies - Entrust Express 4.0
IBM - IBM Security Software Development Kit 1.0
IBM - IBM SSLite 1.0
Lotus - Lotus Notes and Domino
Microsoft - Outlook Express 4.0
Microsoft - Outlook 98
Netscape - Communicator 4.01
Novell - GroupWise 5.5 Enh. Pack 2
The OpenSSL Project - OpenSSL 0.9.5
RSA Security - RSA BSAFE S/MIME-C 1.0

Later versions of the software may be "approved," but there are no test results at:
http://www.rsasecurity.com/standards/smime/interop_center.html