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What is an email header?

Email messages consist of different parts, including:

  • The envelope
  • The body
  • The header

A full and valid email header provides a detailed log of the network path taken by the email between the mail sender's server and the mail receiver's server. 

Header fields
Headers can include standard header fields, as well as non-standard header fields such as X-headers. Information in the headers includes the From, To, and Reply-to addresses, authentication results, spam filter data, and more. Header fields are defined in the Request For Comments (RFCs) for email: 

Headers are normally not shown in their raw and full form by email clients. Usually, only some lines, such as the Subject Line, From, To, and Date, are displayed to the recipient. However, most email clients offer the option to make the full header easily visible. MX Toolbox provides information on how to view headers in various email programs.

Header use
Email headers can be used to identify problems with the email or help to discover the sources of unsolicited email.  Email headers should always be read from bottom to top when using them for troubleshooting purposes.

Example of a full email header

Return-Path: <[email protected]>
X-SpamCatcher-Score: 1 [X]
Received: from [123.456.78.9] (HELO domain.edu)
    by jh1.otherdomain.edu (CommuniGate Pro SMTP 4.1.8)
    with ESMTP-TLS id 61258719 for [email protected]; Mon, 23 Aug 2004 11:40:10 -0400
Message-ID: <[email protected]>
Date: Mon, 23 Aug 2004 11:40:36 -0400
From: ABC University<[email protected]>
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.0.1) Gecko/20020823 Netscape/7.0
X-Accept-Language: en-us, en
MIME-Version: 1.0
To: Jon Doe <[email protected]>
Subject: Business Development Meeting
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Transfer-Encoding: 7bit

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