What are email headers?

About

Email headers are a list of key pair value added to the beginning of email message that acts as metadata.

They specify the:

• where: recipient,
• the who: the sender,
• and the from: the author

Example

An email full with headers where only the last line is not an header but the body

Delivered-To: [email protected]
Date: Fri, 12 Jun 2020 21:08:18 +0200
To: [email protected]
Subject: Without hosts dkim test !
User-Agent: Heirloom mailx 12.5 7/5/10
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Message-Id: <[email protected]>
From: [email protected] (root)
cc: [email protected]
bcc: [email protected]

This is the body of the email

List

• the address
• the format (mime) of the body
• the dkim
• the Received (for the routing)
• the user-agent (the client)
• the message id
• the return-path for bounce message
• the Delivered-To is the final recipient, ie the inbox
• the Content-Type header defines the type of the body
• and more

Who adds them?

• The client will add headers that defines information about the message
• Every server will add for tracing:
  • at minimal the received header
  • and custom headers X-name.

Format

Header fields are lines composed of:

• a field name,
• followed by a colon (“:”),
• followed by a field body,
• and terminated by CRLF (end of line)

Analyze

You can analyze them (ie format them) with the message header toolbox

Specification

• RFC2822 header field