Structure of an email
An email has three basic parts:
- 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 a 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 proper, 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. Each one has the following form:
Name: Value An email includes at least the three following headers:
It may contain the following optional fields:
- From: The sender's email address
- To: The recipient's email address
- Date: The date when the email was sent
- Received: Various information about the intermediary servers and the date when the message was processed.
- Reply-To: A reply address.
- Subject: The message's subject
- Message-ID: A unique identification for the message.
- 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 so called because their name must begin with X-.
Some anti-spam software programs mark messages as unwanted using the following header: