Internet Message Format
Overview
What people casually call "email" actually refers more specifically to a text message written in a specified format called "Internet Message Format" or "IMF". After the first IMF specification was published in 1982 there was no looking back. Email took the world by storm. Today it is arguably the most prevalent means of communicating with a distant party.
IMF is very basic, and is limited to only specifying a syntax for text messages. For the transmission of images, audio, or other types of data we need to make use of the MIME specification. Although IMF forms the base standard upon which email is based, email today is rarely used without MIME. We therefore consider for practical purposes that an email message is essentially the equivalent of a MIME message.
Specifications
The specifications for Internet Message Format (IMF) form the basis of what we commonly call "email".
RFC822
RFC822 ("Standard for the Format of ARPA Internet Text Messages") was the original standard that defined the format of an email. It was obsoleted by RFC2822. The definition of an email under this standard was an attempt to take the lessons learned from the ARPANET and extend the use of text messaging to a broader context.
Electronic mail messages are defined as having contents and an envelope. The contents consist of header fields and, optionally, a body. The body is nothing more than a (potentially empty) sequence of lines of text.
Although this sounds like an extremely simple concept, to get two completely separate systems to agree and understand each other is a surprisingly complex problem that most people today take for granted. Most of RFC822 deals with the nitty-gritty of formatting and parsing this type of text message.
This specification deals with the headers, additionally relating them to the sending and forwarding of messages. The body content is dealt with in [RFC2045]
RFC2822
The standard builds on RFC822, but limits its scope to only the sytax of the message, and obsoletes much of what was defined by RFC822. The envelope was split into a separate specification, [RFC2821].
James Model
While the general description of IMF is not sufficient for building a complex system like James, the technical specifications are unfortunately very messy and overly-complex due to their history and the context in which they were developed.
Since modern-day messaging almost always requires MIME, and since the separation between IMF and MIME is not really useful from a usage perspective, James considers an "email" to be both IMF- an MIME-compliant. For all intents and purposes, James does not consider the concept of IMF in its domain model.