Structure of an Email (Headers and Bodies)

An email has three basic parts. The first is the header, a set of lines containing information about the message's transportation, such as the sender's address, the recipient's address, or timestamps showing when the message was sent by intermediary servers to the transport agents (MTAs), which act as a mail sorting office. The header begins with the From line and is changed each time it passes through an intermediary server. Using headers, you can see the exact path taken by the email and how long it took each server to process.

The message itself, made up of the two following elements: the header fields, a set of lines describing the message's settings, such as the sender, the recipient, the date, etc.

An email includes at least the three following headers: From, showing the sender's email address; To, showing the recipient's email address; and Date, showing the date when the email was sent.

It may contain the following optional fields: Received, showing various information about the intermediary servers and the date when the message was processed; <bold>Reply-To, showing a reply address; subject showing the message's subject; message-ID, showing a unique identification for the message; or the message body, containing the message, separated from the header by a line break.

An email is made up of lines of displayable 7-bit US-ASCII characters. Each line has, at most, 76 characters for compatibility reasons and ends with the characters CRLF (\r\n).

Concept of a Header

It is important to note that header data is no guarantee of when the message was sent or who sent it.

Additional personalized headers (called X-headers) can be set in order to provide the appropriate information. X-headers are called such because their name must begin with X-.

For example, some anti-spam software programs mark messages as unwanted using the following header:
X-Spam-Status: Yes

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