Digest Format: Digest message format
The standard format for bundling multiple email messages into a single digest.
Wancho, F.J., "Digest message format", RFC 1153, DOI 10.17487/RFC1153, April 1990, <https://www.rfc-editor.org/info/rfc1153>.
@misc{rfc1153,
series = {Request for Comments},
number = 1153,
howpublished = {RFC 1153},
publisher = {RFC Editor},
doi = {10.17487/RFC1153},
url = {https://www.rfc-editor.org/info/rfc1153},
author = {F.J. Wancho},
title = {{Digest message format}},
pagetotal = 4,
year = 1990,
month = apr,
} RFC text
Contents
Network Working Group F. Wancho
Request for Comments: 1153 WSMR
April 1990
Digest Message Format
Status of this Memo
This memo describes the de facto standard Digest Message Format.
This is an elective experimental protocol. Distribution of this memo
is unlimited.
Background
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.
Description
A digest message is a conventional message consisting of a header and
body conforming to RFC 822 as clarified in RFC 1123. There is no
fixed size. Limitations may exist in intermediate mail gateways
which restrict the size. The typical digest size is 15,000
characters.
The header of a digest message should identify the digest in the
Subject line by listname, the key word, Digest, the volume number
(usually a sequential number either starting at 1 or the last two
digits of the year and incremented by one starting with the first
issue of the next calendar year), and an issue number starting at one
for the first issue of a new calendar year.
The body of a digest message must consist of a Preamble, one or more
enclosed messages, and a Trailer.
The Preamble usually contains a table of contents consisting of the
subject line contents of the enclosed messages, usually indented or
centered, and also may contain brief administrative or other
announcements.
The Preamble must be separated from the remainder of the message by a
line of 70 hyphens followed by a blank line.
Each enclosed message is a conventional message consisting of a
header and body, separated by a blank line. If they exist in the
original message header, the following lines must be retained as-is
in the reconstructed header: Date:, From:, To:, Cc:, Subject:,
Message-ID:, and Keywords:, rearranged to appear in that order.
Retaining the Summary: line is optional. Lines include continuation
lines as defined in the RFCs. All other header lines should be
discarded, especially Received lines. All leading and trailing blank
lines should be removed from the message body. The message body may
be scanned to replace with a blank the first character of any lines
of exactly and only 30 hyphens.
Each enclosed message must be separated from the the remainder of the
digest message by a blank line before and after a line of 30 hyphens.
The Trailer immediately follows the blank line of message separator
following the last enclosed message. The Trailer consists of two
lines. The first line must begin with the words, End of, followed by
the listname a blank and the word Digest which is usually followed by
volume and issue number on the same line. The second and last line
of the Trailer and the entire message is a line of asterisks serving
to underline the line immediately above it.
Example
The following example serves as a template for a digest message
conforming to this memo. It contains a header, and a body consisting
of a Preamble, two enclosed messages, and a Trailer. fqhn means
Fully Qualified Host Name. The Date: lines are examples only.
Date: ddd, dd mmm yy hh:mm:ss zzz
From: listname-REQUEST@fqhn
Reply-To: listname@fqhn
Subject: listname Digest Vyy #nn
To: listname@fqhn
listname Digest ddd, dd mmm yy Volume yy : Issue nn
Today's Topics:
Message One Subject
Message Two Subject
Administrivia:
Messages from the list moderator or maintainer go here.
----------------------------------------------------------------------
Date: ddd, dd mmm yy hh:mm:ss zzz
From: Joe User <username@fqhn>
Subject: Message One Subject
This is the message body of the first message.
Joe
------------------------------
Date: ddd, dd mmm yy hh:mm:ss zzz
From: Jane User <username@fqhn>
Subject: Message Two Subject
This is the body of message two.
Jane
------------------------------
End of listname Digest Vyy Issue #nn
************************************
Security Considerations
Security issues are not discussed in this memo.
Author's Address
Frank J. Wancho
USAISC-White Sands
ASQNC-TWS-SS-C (F. Wancho, Building 1512)
White Sands Missile Range, NM 88002-5506
Phone: (505) 678-3009
Email: WANCHO@WSMR-SIMTEL20.ARMY.MIL
AI ELI5
On this page
§1Why Digests Exist
High-traffic large mailing lists began appearing on the internet in the mid-1970s. Mailing list moderators developed digest messages as a way to reduce network overhead and mail server load. By bundling many messages into one composite message, bandwidth is conserved and mailer load is proportional to the number of digests rather than the number of individual messages. The digest format became the de facto standard for this purpose, predating RFC 934's generic message encapsulation and remaining the preferred format for digest messages.
When many people email a group, the mail system gets overwhelmed. Putting all the emails together into one big message uses less electricity and is faster for everyone.
§2The Digest Structure
A digest message conforms to RFC 822 (the standard for internet mail) and has no fixed size, though intermediate mail gateways may impose limits. The typical digest is around 15,000 characters. The digest header should identify the digest in the Subject line by listing the mailing list name, the word "Digest", a volume number, and an issue number. The message body must consist of three parts: a Preamble containing a table of contents and administrative announcements, one or more enclosed messages, and a Trailer marking the end. Each enclosed message retains specific header lines in a defined order (Date, From, To, Cc, Subject, Message-ID, Keywords), and removes Received lines and other extraneous headers. Messages are separated by blank lines and a line of exactly 30 hyphens.
The big envelope has a table of contents at the top, then each letter inside, each separated by a line of dashes, and at the very end a note saying this is the end of the digest.
§3Reconstructing Individual Messages
The digest format allows moderators to edit content and reformat messages while preserving the essential message structure needed for reconstruction. Software can separate (undigestify) the composite message back into individual messages by identifying the separator lines and extracting each enclosed message. By retaining the key header fields and removing only Received lines and blank lines, the original message context is preserved. The format also allows optional replacement of lines of exactly 30 hyphens in message bodies with a blank character to prevent false matches with the message separator delimiter.
The format makes it easy to take the big envelope apart again into separate emails. The important information from each letter is kept so the undigestifier software can put them back together.