S/MIME
See http://www.imc.org/smime-pgpmime.html, and read rfc2015

S/MIME v3 encrypted mails:
Content-Type: application/pkcs7-mime; charset="iso-8859-1"
content-transfer-encoding: 7bit

S/MIME v3 signed mails:
Content-Type: multipart/signed; charset="iso-8859-1" (may be CMS format too...)
content-transfer-encoding: 7bit
