The format of Internet e-mail messages is defined in RFC 2822 and a series of RFCs: RFC 2045 through RFC 2049, collectively called Multipurpose Internet Mail Extensions (MIME). Although as of July 13 2005, RFC 2822 is technically a proposed IETF standard and the MIME RFCs are draft IETF standards, these documents are the de facto standards for the format of Internet e-mail.
Prior to the introduction of RFC 2822 in 2001 the format described by RFC 822 was the de facto standard for Internet e-mail for nearly two decades; it is still the official IETF standard.
This format is used by SpamAssassin as its input/output format, since it's the standard format for e-mail message interchange.
How to Retrieve an RFC-822 Format Message from an E-mail
The folks at SpamCop.net maintain an excellent resource addressing the common question of "How do I get my email program to reveal the full, unmodified email?" from many popular e-mail clients for numerous operating systems.
Much of this page was copied from this ASSP wiki page, which in turn used bits from Wikipedia. thanks!