1.5 The Header

Let's examine the header in more detail.

From you@Here.US.EDU  Fri Dec 13 08:11:44 1996
Received: (from you@localhost) by Here.US.EDU (8.8.4/8.8.4)
       id AA04599 for you; Fri, 13 Dec 96 08:11:44 -0700
Date: Fri, 13 Dec 96 08:11:43
From: you@Here.US.EDU (Your Full Name)
Message-Id: <9631121611.AA02124@Here.US.EDU>
To: you                                            may be something else (see Section 34.8.43, 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. There are many types of header lines that can appear in a mail message. Some are mandatory, some are optional, and some may appear many times. Those that appeared in the message that you mailed to yourself were all mandatory. 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 and 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: [2] line shows a list of one or more recipients. (Multiple recipients would be separated with commas.)

[2] 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 Section 34.8.43).

A complete list of all header lines that are of importance to sendmail is presented in Chapter 35, Headers. The important concept here is that the header precedes, and is separate from, the body in all mail messages.