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Mail Tester
Email Headers Explained

Every email you send will have email headers. Email headers were designed to share information about the email that was sent, so the receiving email server can validate, authenticate, interpret, and implement certain functionality based on the email headers that exist. Some email header are completely informational, some are required, and some are important to have in order to have your emails delivered properly.

If you're sending an email, it must contain the following headers (from, date, to, message-id, and subject). Keep in mind, that the value associated with some of these email headers can be blank, but they must be present and should only appear 1 time.

Other others like "received", are added by the receiving email servers, and can appear multiple times as your email is transferred to different places, before reaching it's final destination. 

Email headers like "dkim-signature" is an authentication type header that allows the receiving mail server to validate if the message was tampered with.

The "List-Unsubscribe" header is optional that email senders can include in the header portion of the messages they send. The receiving mail client then displays the information in this header to allow the user to easily un-subscribed to the email newsletter or communication

If you ever wondered what headers are on the emails you send out, you can use our Mail Tester Pro Tool it will display all the header information contained within the email you sent. It will also do a basic validation check to ensure all the required headers are present and in the right quantity. Below is the sample section of the report concerning header information. The entire report is based off validating and authenticating information found in the headers.

Read out next blog in the Mail Tester Series: Mail Tester - Understanding MX Records, PTR Records, and Reverse PTR Records AKA rDNS.

The Mail Tester Pro Report - Header Information Section

Publication: RFC 822
Header Information
Name Value
return-path <mailtest@unlocktheinbox.com>
received from mail.unlocktheinbox.com (mail.unlocktheinbox.com []) by unlocktheinbox.com with SMTP; Thu, 5 Apr 2012 22:56:19 -0400
dkim-signature v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=unlocktheinbox.com; s=secure; h=Content-Transfer-Encoding:Content-Type:MIME-Version:To:From:Subject:Date:Message-ID; bh=g3zLYH4xKxcPrHOD18z9YfpQcnk/GaJedfustWU5uGs=; b=V9/t0UnN278ioLmuMkylTQYQnYs3Si59CEMbSkgoxBxOgTL5A1J7GwZLH2POnGvVN107izPGI5ekUxONZRG19a+F/8owB675w4kc6ragbvNX3Op5MFBquy0WlbLL5GlJ;
domainkey-signature a=rsa-sha1; q=dns; c=nofws; s=secure; d=unlocktheinbox.com; h=Message-ID:Date:Subject:From:To:User-Agent:MIME-Version:Content-Type:Content-Transfer-Encoding:X-Priority:Importance; b=dSMhrkgARCXZqVdkDq/1/irPFgXoAUllgC0elg5PUAMsWHp8yQ9sq
received from localhost.localdomain ([] helo=mail.unlocktheinbox.com) by mail.unlocktheinbox.com with esmtpa (Exim 4.69) (envelope-from) id 1SFzLW-00013Y-MV for mailtest@unlocktheinbox.com; Thu, 05 Apr 2012 22:55:54 -0400
received from ([]) (proxying for (SquirrelMail authenticated user mailtest@unlocktheinbox.com) by mail.unlocktheinbox.com with HTTP; Thu, 5 Apr 2012 21:56:18 -0500
message-id <cafd4302c5235420@mail.unlocktheinbox.com>
date Thu, 5 Apr 2012 21:56:18 -0500
subject This is a test of Unlock The Inbox Mail Tester.
from "Mail Tester" <mailtest@unlocktheinbox.com>
to mailtest@unlocktheinbox.com
user-agent SquirrelMail/1.4.22
mime-version 1.0
content-type text/plain;charset=iso-8859-1
content-transfer-encoding 8bit
x-priority 3 (Normal)
importance Normal
x-antiabuse This header was added to track abuse, please include it with any abuse report
x-antiabuse Primary Hostname - mail.unlocktheinbox.com
x-antiabuse Original Domain - unlocktheinbox.com
x-antiabuse Originator/Caller UID/GID - [47 12] / [47 12]
x-antiabuse Sender Address Domain - unlocktheinbox.com

Publication: RFC 5322
Header Counts
Name Count Result
From 1 Passed
Date 1 Passed
To 1 Passed
CC 0 Passed
BCC 0 Passed
Reply-To 0 Passed
In-Reply-To 0 Passed
Messsage-ID 1 Passed
Subject 1 Passed
References 0 Passed
Sender 0 Passed
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