Sendmail, 3rd Edition

Basic Parts of a Mail Message

In this section, we will examine the three parts that make up a mail message: the header, body, and envelope. But before we do, we must first demonstrate how to run sendmail by hand so that you can see what a message’s parts look like.

Run sendmail by Hand

Most users do not run sendmail directly. Instead, they use one of many MUAs to compose a mail message. Those programs invisibly pass the mail message to sendmail, creating the appearance of instantaneous transmission. The sendmail program then takes care of delivery in its own seemingly mysterious fashion.

Although most users don’t run sendmail directly, it is perfectly legal to do so. You, like many system managers, might need to do this to track down and solve mail problems.

Here’s a demonstration of one way to run sendmail by hand. First create a file named sendstuff with the following contents:

This is a one-line message.

Second, mail this file to yourself with the following command line, where you is your login name:

% /usr/sbin/sendmail you<sendstuff 

Here, you run sendmail directly by specifying its full pathname.^[2] When you run sendmail, any command-line arguments that do not begin with a - character are considered to be the names of the people to whom you are sending the mail message.

The <sendstuff sequence causes the contents of the file that you have created (sendstuff) to be redirected into the sendmail program. The sendmail program treats everything it reads from its standard input (up to the end of the file) as the mail message to transmit.^[3]

Now view the mail message you just sent. How you do this will vary. Many users just type mail to view their mail. Others use the mh(1) package and type inc to receive and show to view their mail. No matter how you normally view your mail, save the mail message you just received to a file. It will look something like this:

From [e-mail]  Fri Dec 13 08:11:44 2002
Received: (from you@localhost)
       by Here.US.EDU (8.12.7/8.12.7)
       id d8BILug12835 for you; Fri, 13 Dec 2002 08:11:44 -0600 (MDT)
Date: Fri, 13 Dec 2002 08:11:43
From: [e-mail] (Your Full Name)
Message-Id: [e-mail]>
To: you   ← might be something else (see NoRecipientAction)
This is a one-line message.

The first thing to note is that this file begins with seven lines of text that were not in your original message. Those lines were added by sendmail and your local delivery program and are called the header.

The last line of the file is the original line from your sendstuff file. It is separated from the header by one blank line. The body of a mail message comes after the header and consists of everything that follows the first blank line (see Figure 1-1).

Figure 1-1. Every mail message is composed of a header and a body

Ordinarily, when you send mail with your MUA, the MUA adds a header and feeds both the header and the body to sendmail. This time, however, you ran sendmail directly and supplied only a body; the header was added by sendmail.

The Header

Let’s examine the header in more detail.

From [e-mail]  Fri Dec 13 08:11:44 2002
Received: (from you@localhost)
       by Here.US.EDU (8.12.7/8.12.7)
       id d8BILug12835 for you; Fri, 13 Dec 2002 08:11:44 -0600 (MDT)
Date: Fri, 13 Dec 2002 08:11:43
From: [e-mail] (Your Full Name)
Message-Id: [e-mail]>
To: you     ← might be something else (see NoRecipientAction)

Notice that most header lines start with a word followed by a colon. Each word tells what kind of information the rest of the line contains. Many types of header lines can appear in a mail message. Some are mandatory, some are optional, and some can appear many times. Those that appeared in the message you mailed to yourself were all mandatory.^[4] That’s why sendmail added them to your message. The line starting with the five characters "From " (the fifth character is a space) is added by some programs (such as /bin/mail) but not by others (such as mh).

A Received: line is added each time a machine receives the mail message. (If there are too many such lines, the mail message will bounce—because it is probably in a loop—and will be returned to the sender as failed mail.) The indented line is a continuation of the line above, the Received: line. The Date: line gives the date and time when the message was originally sent. The From: line lists the email address and the full name of the sender. The Message-ID: line is like a serial number in that it is guaranteed to uniquely identify the mail message. And the To:^[5] line shows a list of one or more recipients. (Multiple recipients would be separated with commas.)

A complete list of all header lines that are of importance to sendmail is presented in Chapter 25. The important concept here is that the header precedes, and is separate from, the body in all mail messages.

The Body

The body of a mail message consists of everything following the first blank line to the end of the file. When you sent your sendstuff file, it contained only a body. Now, edit the file sendstuff and add a small header:

Subject: a test         ←add
                        ←add
This is a one-line message.

The Subject: header line is optional. The sendmail program passes it through as is. Here, the Subject: line is followed by a blank line and then the message text, forming a header and a body. Note that a blank line must be truly blank. If you put space or tab characters in it, thus forming an “empty-looking” line, the header will not be separated from the body as intended.

Send this file to yourself again, running sendmail by hand as you did before:

% /usr/sbin/sendmail you<sendstuff

Notice that our Subject: header line was carried through without change:

From [e-mail]  Fri Dec 13 08:11:44 2002
Received: (from you@localhost)
       by Here.US.EDU (8.12.7/8.12.7)
       id d8BILug12835 for you; Fri, 13 Dec 2002 08:11:44 -0600 (MDT)
Date: Fri, 13 Dec 2002 08:11:43
From: [e-mail] (Your Full Name)
Message-Id: [e-mail]>
Subject: a test             ←note
To: you

This is a one-line message.

The Envelope

So that it can more easily handle delivery to diverse recipients, the sendmail program uses the concept of an envelope. This envelope is analogous to the physical envelopes that are used for post office mail. Imagine you want to send two copies of a document, one to your friend in the office next to yours and one to a friend across the country:

To: friend1, friend2@remote

After you photocopy the document, you stuff each copy into a separate envelope. You hand one envelope to a clerk, who carries it next door and hands it to friend1 in the next office. This is like delivery on your local machine. The clerk drops the other copy in the slot at the corner mailbox, and the post office forwards that envelope across the country to friend2@remote. This is like sendmail transporting a mail message to a remote machine.

To illustrate what an envelope is, consider one way in which sendmail might run /usr/lib/mail.local, a program that performs local delivery:

                   deliver to friend1's mailbox
                    ↓
/usr/lib/mail.local -d friend1     ←sendmail runs
                          ↑the envelope recipient

Here sendmail runs /usr/lib/mail.local with a -d, which tells /usr/lib/mail.local to append the mail message to friend1’s mailbox.

Information that describes the sender or recipient, but is not part of the message header, is considered envelope information. The two might or might not contain the same information (a point we’ll gloss over for now). In the case of /usr/lib/mail.local, the email message showed two recipients in its header:

To: friend1, friend2@remote    ←the header

But the envelope information that is given to /usr/lib/mail.local showed only one (the one appropriate to local delivery):

-d friend1     ←specifies the envelope

Now consider the envelope of a message transported over the network. When sending network mail, sendmail must give the remote site the envelope-sender address and a list of recipients separate from and before it sends the mail message (header and body). Figure 1-2 shows this in a greatly simplified conversation between the local sendmail and the remote machine’s sendmail.

Figure 1-2. A simplified conversation

The local sendmail tells the remote machine’s sendmail that there is mail from you (the envelope-sender) and for friend2@remote. It conveys this envelope-sender and recipient information separate from and before it transmits the mail message that contains the header. Because this information is conveyed separately from the message header, it is called the envelope.

Only one recipient is listed in the envelope, whereas two were listed in the message header:

To: friend1, friend2@remote

The remote machine should not need to know about the local user, friend1, so that bit of recipient information is excluded from the envelope.

A given mail message can be sent by using many different envelopes (like the two here), but the header will be common to them all. But note that the headers of a message don’t necessarily reflect the actual envelope. You witness such mismatches whenever you receive a message from a mailing list, or receive a spam message.

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^[2] That path might be different on your system. If so, substitute the correct pathname in all the examples that follow. For example, try looking for sendmail in /usr/lib or /usr/ucblib.

^[3] We are fudging for simplicity here. If the file contains a line that contains only a single dot, that line will be treated as if it marks the end of the file. If you need to include such a line as part of literal input, use the IgnoreDots options (IgnoreDots).

^[4] We are fudging for simplicity. The Message-ID: header is not strictly mandatory.

^[5] Depending on how the NoRecipientAction option was set, this could be an Apparently-To: header, a Bcc: header, or even a To: header followed by an "undisclosed-recipients:;" (see NoRecipientAction).