Chapter 17. Electronic Mail

Table of Contents
What Is a Mail Message?
How Is Mail Delivered?
Email Addresses
How Does Mail Routing Work?
Configuring elm

Electronic mail transport has been one of the most prominent uses of networking since the first networks were devised. Email started as a simple service that copied a file from one machine to another and appended it to the recipient's mailbox file. The concept remains the same, although an ever-growing net, with its complex routing requirements and its ever increasing load of messages, has made a more elaborate scheme necessary.

Various standards of mail exchange have been devised. Sites on the Internet adhere to one laid out in RFC-822, augmented by some RFCs that describe a machine-independent way of transferring just about anything, including graphics, sound files, and special characters sets, by email.[1] CCITT has defined another standard, X.400. It is still used in some large corporate and government environments, but is progressively being retired.

Quite a number of mail transport programs have been implemented for Unix systems. One of the best known is sendmail, which was developed by Eric Allman at the University of California at Berkeley. Eric Allman now offers sendmail through a commercial venture, but the program remains free software. sendmail is supplied as the standard mail agent in some Linux distributions. We describe sendmail configuration in Chapter 18.

Linux also uses Exim, written by Philip Hazel of the University of Cambridge. We describe Exim configuration in Chapter 19.

Compared to sendmail, Exim is rather young. For the vast bulk of sites with email requirements, their capabilities are pretty close.

Both Exim and sendmail support a set of configuration files that have to be customized for your system. Apart from the information that is required to make the mail subsystem run (such as the local hostname), there are many parameters that may be tuned. sendmail 's main configuration file is very hard to understand at first. It looks as if your cat has taken a nap on your keyboard with the shift key pressed. Exim configuration files are more structured and easier to understand than sendmail 's. Exim, however, does not provide direct support for UUCP and handles only domain addresses. Today that isn't as big a limitation as it once might have been; most sites stay within Exim's limitations. However, for most sites, the work required in setting up either of them is roughly the same.

In this chapter, we deal with what email is and what issues administrators have to deal with. Chapter 18 and Chapter 19 provide instructions on setting up sendmail and Exim and for the first time. The included information should help smaller sites become operational, but there are several more options and you can spend many happy hours in front of your computer configuring the fanciest features.

Toward the end of this chapter we briefly cover setting up elm, a very common mail user agent on many Unix-like systems, including Linux.

For more information about issues specific to electronic mail on Linux, please refer to the Electronic Mail HOWTO by Guylhem Aznar,[2] which is posted to comp.os.linux.answers regularly. The source distributions of elm, Exim, and sendmail also contain extensive documentation that should answer most questions on setting them up, and we provide references to this documentation in their respective chapters. If you need general information on email, a number of RFCs deal with this topic. They are listed in the bibliography at the end of the book.

What Is a Mail Message?

A mail message generally consists of a message body, which is the text of the message, and special administrative data specifying recipients, transport medium, etc., like what you see when you look at a physical letter's envelope.

This administrative data falls into two categories. In the first category is any data that is specific to the transport medium, like the address of sender and recipient. It is therefore called the envelope. It may be transformed by the transport software as the message is passed along.

The second variety is any data necessary for handling the mail message, which is not particular to any transport mechanism, such as the message's subject line, a list of all recipients, and the date the message was sent. In many networks, it has become standard to prepend this data to the mail message, forming the so-called mail header. It is offset from the mail body by an empty line.[3]

Most mail transport software in the Unix world use a header format outlined in RFC-822. Its original purpose was to specify a standard for use on the ARPANET, but since it was designed to be independent from any environment, it has been easily adapted to other networks, including many UUCP-based networks.

RFC-822 is only the lowest common denominator, however. More recent standards have been conceived to cope with growing needs such as data encryption, international character set support, and MIME (Multipurpose Internet Mail Extensions, described in RFC-1341 and other RFCs).

In all these standards, the header consists of several lines separated by an end-of-line sequence. A line is made up of a field name, beginning in column one, and the field itself, offset by a colon and white space. The format and semantics of each field vary depending on the field name. A header field can be continued across a newline if the next line begins with a whitespace character such as tab. Fields can appear in any order.

A typical mail header may look like this:
Return-Path: <>
Received: ( [])
    by (8.9.3/8.9.3/Debian 8.9.3-6) with ESMTP id WAA04654
    for <>; Sun, 30 Jan 2000 22:30:01 +1100
Received: from ph10 (helo=localhost) by with local-smtp
    (Exim 3.13 #1) id 12EsYC-0001eF-00; Sun, 30 Jan 2000 11:29:52 +0000
Date: Sun, 30 Jan 2000 11:29:52 +0000 (GMT)
From: Philip Hazel <>
Reply-To: Philip Hazel <>
To: Terry Dawson <>, Andy Oram <>
Subject: Electronic mail chapter
In-Reply-To: <>
Message-ID: <>

Usually, all necessary header fields are generated by the mailer interface you use, like elm, pine, mush, or mailx. However, some are optional and may be added by the user. elm, for example, allows you to edit part of the message header. Others are added by the mail transport software. If you look into a local mailbox file, you may see each mail message preceded by a “From” line (note: no colon). This is not an RFC-822 header; it has been inserted by your mail software as a convenience to programs reading the mailbox. To avoid potential trouble with lines in the message body that also begin with “From,” it has become standard procedure to escape any such occurrence by preceding it with a > character.

This list is a collection of common header fields and their meanings:


This contains the sender's email address and possibly the “real name.” A complete zoo of formats is used here.


This is a list of recipient email addresses. Multiple recipient addresses are separated by a comma.


This is a list of email addresses that will receive “carbon copies” of the message. Multiple recipient addresses are separated by a comma.


This is a list of email addresses that will receive “carbon copies” of the message. The key difference between a “Cc:” and a “Bcc:” is that the addresses listed in a “Bcc:” will not appear in the header of the mail messages delivered to any recipient. It's a way of alerting recipients that you've sent copies of the message to other people without telling them who those others are. Multiple recipient addresses are separated by a comma.


Describes the content of the mail in a few words.


Supplies the date and time the mail was sent.


Specifies the address the sender wants the recipient's reply directed to. This may be useful if you have several accounts, but want to receive the bulk of mail only on the one you use most frequently. This field is optional.


The organization that owns the machine from which the mail originates. If your machine is owned by you privately, either leave this out, or insert “private” or some complete nonsense. This field is not described by any RFC and is completely optional. Some mail programs support it directly, many don't.


A string generated by the mail transport on the originating system. It uniquely identifies this message.


Every site that processes your mail (including the machines of sender and recipient) inserts such a field into the header, giving its site name, a message ID, time and date it received the message, which site it is from, and which transport software was used. These lines allow you to trace which route the message took, and you can complain to the person responsible if something went wrong.


No mail-related programs should complain about any header that starts with X-. It is used to implement additional features that have not yet made it into an RFC, or never will. For example, there was once a very large Linux mailing list server that allowed you to specify which channel you wanted the mail to go to by adding the string X-Mn-Key: followed by the channel name.



Read RFC-1437 if you don't believe this statement!


Guylhem can be reached at


It is customary to append a signature or .sig to a mail message, usually containing information on the author along with a joke or a motto. It is offset from the mail message by a line containing “--” followed by a space.