The Header

Let’s examine the header in more detail:

From you@Here.US.EDU  Fri Dec 14 08:11:44 2007
Received: (from you@localhost)
       by Here.US.EDU (8.12.7/8.12.7)
       id d8BILug12835 for you; Fri, 14 Dec 2007 08:11:44 −0600 (MDT)
Date: Fri, 14 Dec 2007 08:11:43
From: you@Here.US.EDU (Your Full Name)
Message-Id: 200712141511.d872mLW24467@Here.US.EDU>
To: you                              ← might be something else (see §24.9.81 on
page 1060)

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.[7] 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 ...

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