Headers Provide Routing Information

Besides the most common identifications (from, to, date, subject), email headers also provide information on the route an email takes as it is transferred from one computer to another. As mentioned earlier, mail transfer agents (MTA) facilitate email transfers. When an email is sent from one computer to another it travels through a MTA. Each time an email is sent or forwarded by the MTA, it is stamped with a date, time and recipient. This is why some emails, if they have had several destinations, may have several RECEIVED headers: there have been multiple recipients since the origination of the email. In a way it is much like the same way the post office would route a letter: every time the letter passes through a post office on its route, or if it is forwarded on, it will receive a stamp. In this case the stamp is an email header.

When viewed in their entirety, these multiple recipient headers will look like this in an email:

Received: from tom.bath.dc.uk ([138.38.32.21] ident=yalrla9a1j69szla2ydr)
by steve.wrath.dc.uk with esmtp (Exim 3.36 #2)id 19OjC3-00064B-00
for example_to@imaps.bath.dc.uk; Sat, 07 Jun 2005 20:17:35 +0100

Received: from write.example.com ([205.206.231.26])
by tom.wrath.dc.uk with esmtp id 19OjBy-0001lb-3V
for example_to@bath.ac.uk; Sat, 07 Jun 2005 20:17:30 +0100

Received: from master.example.com (lists.example.com [205.206.231.19])
by write.example.com (Postfix) with QMQP
id F11418F2C1; Sat, 7 Jun 2005 12:34:34 -0600 (MDT)
In the example shown above, there are three Received: stamps. Reading from the bottom upwards, you can see who sent the message first, next and last, and you can see when it was done. This is because every MTA that processed the email message added a Received: line to the email's header. These Received: lines provide information on where the message originated and what stops it made (what computers) before reaching its final destination. As the example shows, these Received: lines provide the email and IP address of each sender and recipient. They also provide the date and time of each transfer. The lines also indicate if the email address was part of an email list. It is all this information that is valued by computer programmers and IT department associates when making efforts to track and stop SPAM email message. And it is this information that arguable makes headers the most important part of an email.