Content Header

The three components of an email are the envelope, the content header and the body of a message. The envelope is completely transparent to either the creator or the recipient of a message as it is an internal process for routing. The body is the message itself contained in the email. These are always preceded by the content text or content header.

The content header contains or identifies specific routing information of a message that includes the sender of the message, the recipient, date, and subject line. Some of the information included in the content header are required and are automatically generated, which are the ‘from’ and ‘to’ line, and the date, while other information is optional but commonly included like the subject line. A typical content header looks like this:

Return-Path: <sender_from@xyz.com>

X-SpamCatcher-Score: 1 [X]

Received: from [130.179.10.2099] (HELO xyz.com)

    by xy4.xyz.com (CommuniGate Pro SMTP 4.1.8)

    with ESMTP-TLS id 61258719 for mail_to@cde.xyz.com; Mon, 23 Aug 2012 11:40:10 -0400

Message-ID: <4129F3CA.2020509@xyz.com>

Date: Mon, 15 Aug 2010 12:30:06 -0700

From: Your Friend < sender_from@xyz.com >

User-Agent: Thunderbird/5.0 (Windows; U; Windows XP 8.1; en-US; rv:1.0.1) Gecko/20020823 Thunderbird/5.0

X-Accept-Language: en-us, en

MIME-Version: 1.0

To: Your Address <sender_to@xyz.com>

Subject: Our lunch date

Content-Type: text/plain; charset=us-ascii; format=flowed

Content-Transfer-Encoding: 7bit

In addition to the most common identifications (from, to, date, subject), content headers also identify information for the route the email takes from one computer to another. Each time an email is sent or forwarded it is done so by a mail transfer agent (MTA), which stamps it with a date, time and recipient. This is why some emails, if they have had several destinations, may have several RECEIVED headers, which are the multiple recipients since the origination of the email. This is the same way in which the post office would route a letter every time it passes through a different post office on the route to the one it was originally addressed to.

Content headers have evolved over the years to become known as Multipurpose Internet Mail Extensions (MIME) and now are an Internet standard that extends the format of email to include non-ASCII character sets, message bodies with multiple parts, and support non-text attachments, such as images and PDF files.

For much of all human-written Internet email and a large proportion of automated email, messages are transmitted via Simple Mail Transfer Protocol (SMTP) in MIME format and for this reason is most generally known as SMTP/MIME email. Other content types that are defined by MIME standards are also used beyond email to include HTTP communication protocols for the web as used in HTML email newsletters, which are much more sophisticated communications.

MIME defines mechanisms for sending text in languages other than English using character encodings other than ASCII, and 8-bit binary content such as files containing images, sounds, video, and executable files. Mapping messages into and out of MIME format is typically done automatically by an email client or by mail servers when sending or receiving Internet (SMTP/MIME) email.

Older MIME specifications only described mail message structure but did not address the issue of presentation styles. The content-disposition header field was added later to specify a presentation style where inline content-disposition is automatically displayed when the message is opened, or as an attachment that requires some form of human action to open it. The MIME also provides the name of a file, creation or modification date that is used when the recipient saves or stores a file on their computer.