Email Header

How to Read and Analyze the Email Header Fields
and Information about SPF, DKIM, SpamAssassin

 

How to access the Message Header

Most mail clients allow access to the message header. The following list contains a few popular mail and web mail clients. 
Please refer to the manual of your mail client if your mail client is not included in this list.

 

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Standard Message Header Fields

Sample Message Header:

Return-path: <sender@senderdomain.tld>
Delivery-date: Wed, 13 Apr 2011 00:31:13 +0200
(3)Received: from mailexchanger.recipientdomain.tld([ccc.ccc.ccc.ccc])
by mailserver.recipientdomain.tld running ExIM with esmtp
id xxxxxx-xxxxxx-xxx; Wed, 13 Apr 2011 01:39:23 +0200
(2)Received: from mailserver.senderdomain.tld ([bbb.bbb.bbb.bbb] helo=mailserver.senderdomain.tld)
by mailexchanger.recipientdomain.tld with esmtp id xxxxxx-xxxxxx-xx
for recipient@recipientdomain.tld; Wed, 13 Apr 2011 01:39:23 +0200
(1)Received: from senderhostname [aaa.aaa.aaa.aaa] (helo=[senderhostname])
by mailserver.senderdomain.tld with esmtpa (Exim x.xx)
(envelope-from <sender@senderdomain.tld) id xxxxx-xxxxxx-xxxx
for recipient@recipientdomain.tld; Tue, 12 Apr 2011 20:36:08 -0100

Message-ID: <xxxxxxxx.xxxxxxxx@senderdomain.tld>
Date: Tue, 12 Apr 2011 20:36:01 -0100
X-Mailer: Mail Client
From: Sender Name <sender@senderdomain.tld>
To: Recipient Name <recipient@recipientdomain.tld>
Subject: Message Subject

 

 

The From: line, which contains the sender of the message could be faked easily, so you should not rely on this information.

The lines in green contain the routing information, from the senders computer to the recipients mail server.

 

Let's take a closer look at the routing information:

(3) Received: from senderhostname [aaa.aaa.aaa.aaa] (helo=[ senderhostname])
by mailserver.senderdomain.tld with esmtpa (Exim x.xx)
(envelope-from <sender@senderdomain.tld) id xxxxx-xxxxxx-xxxx
for recipient@recipientdomain.tld; Tue, 12 Apr 2011 20:36:08 -0100

(3) The message was sent from the senders computer with the IP address aaa.aaa.aaa.aaa to the mail server of the sender. In many cases the sender IP aaa.aaa.aaa.aaa is a dynamic IP address, e.g. DSL. The IP address gives many information about the sender, the location of the sender and the provider.

 

2) Received: from mailserver.senderdomain.tld ([bbb.bbb.bbb.bbb] helo=mailserver.senderdomain.tld)
by mailexchanger.recipientdomain.tld with esmtp
id xxxxxx-xxxxxx-xx

(2) The message was transferd from the senders mail server with the IP address bbb.bbb.bbb.bbb to the recipients mail-exchanger. The mail-exchanger is the mail server, which accepts incoming messages for a domain.

 

(1) Received: from mailexchanger.recipientdomain.tld([ccc.ccc.ccc.ccc])
by mailserver.recipientdomain.tld running ExIM with esmtp
id xxxxxx-xxxxxx-xxx; Wed, 13 Apr 2011 01:39:23 +0200

(1) The message was finally received by the recipients mail server from the the recipients mail exchanger ccc.ccc.ccc.ccc.

 

This is only a sample, which should show the principles. The message routing can contain much more steps, depending on the used mail provider. It should always be possible to see the sender computer IP address aaa.aaa.aaa.aaa and the sender mail server bbb.bbb.bbb.bbb, if the message was sent from a mail client and a client computer. If the message was sent from a webmail client, then the real IP address of the sender is not included - in this case aaa.aaa.aaa.aaa (if any) will be the IP address of the webmail.

Some might try to fake the routing information, but your mail server should give you a warning that something is not correct during the transfer from the sender mail server bbb.bbb.bbb.bbb to the recipient mail exchanger ccc.ccc.ccc.ccc.

 

Sender Policy Framework (SPF)

The Sender Policy Framework SPF is a framework to prevent sender address forgery.

In easy words (simplified): 
"SPF is used to describe what mail server is allowed to send messages for a domain".
See also: How to enable Sender Policy Framework (SPF) for domain/email

It's used to avoid fake email addresses (as sender email address). The system can detect if the mail server, which wants to send a message to the recipients mail-exchanger, is valid for the senders email address (domain).

The result can be:
Received-SPF: neutral
Received-SPF: pass

If it fails,
Received-SPF: fail
The message should be rejected by the recipient's mail exchanger.

More Information about the Sender Policy Framework (SPF) by OpenSPF.org

DomainKeys Identified Mail (DKIM)

"Domain Keys Identified Mail (DKIM) is a method for associating a domain name to an email, thereby allowing an organization to take responsibility for a message in a way that can be validated by a recipient." ... from Wikipedia.org

In other words (simplified): 
"Some organization (domain) has signed the message and is responsible for it".

It's a good indicator, that the message is NOT a spam message. No spammer would "sign" a message.

SpamAssassin's Header Lines

SpamAssassin is a anti-spam software, which is installed on many mail server. It's a great tool, which gives a detailed report for each message by adding lines and a summary to the message header.

 

Sample for a clean message:

X-Spam-Level: ---
X-Spam-Score: -3.5
X-Spam-Report: Content analysis details: (-3.5 points)
 
pts rule name description 
---- ---------------------- --------------------------------------------------
-1.0 RCVD_IN_DNSWL_LOW RBL: Sender listed at http://www.dnswl.org/, 
low trust [xxx.xxx.xxx.xxx listed in list.dnswl.org]
-2.5 SPF_PASS SPF: sender matches SPF record
-0.0 DKIM_VERIFIED Domain Keys Identified Mail: signature passes verification
 0.0 DKIM_SIGNED Domain Keys Identified Mail: message has a signature
 0.0 RDNS_NONE Delivered to trusted network by a host with no rDNS

A X-Spam-Score <5 means (on most systems) no spam, >5 probably spam and >15 spam. Spam might get deleted immediately or moved to the junk mail folder. Some systems add [SPAM] to the subject, so that these messages could be moved to the junk folder in the mail client using a rule.

This sample shows a clean message, with a negative spam rating! It includes a SPF-pass and the message is DKIM signed.

SpamAssasin does not only check the message sender, it also checks the format and the content of the message.

 

Here is a sample of a spam-message, which should illustrate it:

x-Spam-Level: ++++++++++++++++++++++++++++++++++
X-Spam-Score: 34.7
X-Spam-Report: Content analysis details: (34.7 points) 
 pts rule name description
---- ---------------------- --------------------------------------------------
2.9 RCVD_IN_XBL RBL: Received via a relay in Spamhaus XBL
0.5 RCVD_IN_PBL RBL: Received via a relay in Spamhaus PBL 
...
1.6 URIBL_AB_SURBL Contains an URL listed in the AB SURBL blocklist
2.1 URIBL_WS_SURBL Contains an URL listed in the WS SURBL blocklist 
2.9 URIBL_JP_SURBL Contains an URL listed in the JP SURBL blocklist 
2.5 URIBL_SC_SURBL Contains an URL listed in the SC SURBL blocklist 
0.9 URIBL_RHS_DOB Contains an URI of a new domain (Day Old Bread) 
...
0.0 MISSING_DATE Missing Date: header 
0.1 HTML_MESSAGE BODY: HTML included in message 
2.4 HTML_IMAGE_ONLY_08 BODY: HTML: images with 400-800 bytes of words 
0.7 MIME_HTML_ONLY BODY: Message only has text/html MIME parts 
1.1 HTML_SHORT_LINK_IMG_1 HTML is very short with a linked image 
1.3 SUBJECT_NEEDS_ENCODING SUBJECT_NEEDS_ENCODING 
...

 

This message has a horrible spam-rating of 34.7. It's definitely a spam message.

 

As you can see, the rules have different scores (spam rating), e.g. HTML_MESSAGE has only 0.1, because any rich-text (html) message would match this rule. HTML_IMAGE_ONLY_08 has a higher score of 2.4, because it indicates an "image message", where the sender tries to avoid the content filter by using an image instead of text.

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