The following list details references for the packaging layers for S/MIME signed, encrypted messages:
RFC2616
Hypertext Transfer Protocol: HTTP/1.1
http://www.ietf.org/rfc/rfc2616.txt
RFC2045
MIME Part One: Format of Internet Message Bodies
http://www.ietf.org/rfc/rfc2045.txt
RFC2633
S/MIME Version 3 Message Specification
http://www.ietf.org/rfc/rfc2633.txt
RFC1847
Security Multiparts for MIME: Multipart/Signed and Multipart/Encrypted
http://www.ietf.org/rfc/rfc1847.txt rex
RFC2376
XML Media Types
http://www.ietf.org/rfc/rfc2376.txt
RFC2298
An Extensible Message Format for Message Disposition Notifications
http://www.ietf.org/rfc/rfc2298.txt