http://www.ietf.org/rfc/rfc1153.txt
High traffic volume large mailing lists began to appear on the net in the mid-70s. The moderators of those lists developed a digest message format to enclose several messages into one composite message for redistribution to the mailing list addressees. This format reduces the mailer load in proportion to the number of messages contained within a digest message, and conserves network bandwidth by reducing the size of the headers of the enclosed messages.

This RFC documents the digest message format so that others may follow this format in creating (digestifying) and separating (undigestifying) digest messages to maintain compatibility with the programs expecting this de facto standard. Any editorial functions performed at the discretion of a digest moderator, such as discarding submissions, editing content to correct spelling and punctuation errors, inserting comments, and reformatting paragraphs to conform to width conventions are beyond the scope of this memo.

This memo describes the de facto standard Digest Message Format. It is not meant to supersede nor replace the GENERIC message encapsulation format described in RFC 934. It merely documents a particular message encapsulation format that existed well before RFC 934 was published and continues to be the format of choice for digest messages.