Received

When an email is sent, it will not contain any "Received" header fields. However, as most people know, when an email arrives there are multiple "Received" header fields. Each mail server that relays the message while the email is in transit will add a "Recieved" header field. The "Received" headers allow the tracing of the email delivery process back to the sender. For this reason, spammers often forge these header fields. The header fields are quite difficult to parse, and it is beyond the scope of this article to describe the format of the field contents.

Example:

Received: from alnrmhc12.comcast.net ([206.18.177.52]) by tagtooga.com with MailEnable ESMTP; Fri, 25 Aug 2006 08:23:45 -0400
Received: from ipdmcz0250mia (tagtooga.com[64.65.53.234])
          by comcast.net (alnrmhc12) with SMTP
          id <20060825122349b1200a6bhue>; Fri, 25 Aug 2006 12:23:49 +0000