Mailjet Team


It is very interesting to know how to decipher the contents of a technical header in an email, as it allows the detection of certain deliverability issues. In fact, the ESP‘s provide a whole range of information  that is easily available.

How to read email headers ?


Check your SMTP relay’s SPF / DKIM authentication

Here are two examples of headers to analyse:

1) In Gmail: Authentication-Results:; spf = pass ( domain of designates as permitted sender)  smtp.mail =; dkim =pass (test mode) header.i=@

2) In Authentication-Results:; spf = pass (sender IP is smtp.mailfrom =; dkim = pass (testing mode);x-hmca=pass

In both cases, the same message is sent on two different platforms. A glance is enough to check that everything is fine: the email is properly configured for the SPF: “spf = pass”; the same applies to the DKIM: “dkim = pass”.

If there was a issue, you would have for example “=tempError“, instead of “= pass”.

How to display an email header?

Webmail / client email systems function differently but in general the option is fairly simple to find.

Some Examples:

•;Gmail: Click “Show original”

How to read email headers ?2


• Hotmail: Click …Actions“ then …View message source“

How to read email headers ?3

• Yahoo!: Click …Actions“ then …View Full Header“

How to read email headers ?4

• Outlook: Right click on the email in the inbox and click on …Message Options“.

• Apple Mail: go to …View“ next …Message“ then …Raw Source“.

For a complete list, there is a very good article produced by Media Temple.

Standard Fields of the Email Header

There are 3 types of fields. Please note that an email header is always read from the bottom up as each relay adds its information to the top of the header.

1. Required Fields *

• From *: Email address of the sender

• To: The recipient’s email address

• Date*: the date and time that each email has been sent

• Subject: The subject of the email that has been sent

• Message-ID: A unique identifier of the email to avoid multiple deliveries of the same message.

• Content-type: Describes how the different sections of the message should be displayed (html, text).

2. Optional Fields / Options

• Reply-to: Address to which the email replies must be sent

• Received: Each relay places a “Received” line in the email header, which allows you to trace the email’s path.

• Return-Path: This is the email address to which error messages should be sent, for example, if the destination address does not exist. This email address will receive the IASP.

•Authentication-Results: As already discussed, this is the line that includes the results of the authentication tests (DKIM, SPF).

• List-unsubscribe: This is an email address or a URL that allows you to unsubscribe from a list directly from the webmail interface or email client.

3. Specific Fields relating to different Platforms

• Fields that begin with an X- are custom fields which are not documented in the RFC. They are used for the needs of different platforms sending and receiving emails. Actually, Mailjet suggests the use of X-headers.

* According to RFC 2822 (internet technical recommendations), only the …From“ and …Date“ fields are actually mandatory in emails, but in practice, these fields are never the only two fields.

[ Posted Mon, 18 Feb 2013 17:50:00 ]