Email

An email is an electronic message transmitted over a network from one user to another.

According to Darwin Magazine: Prime Movers, the first email message was sent in 1971 by an engineer named Ray Tomlinson. Before this, you could only send messages to users on a single machine. Tomlinson's breakthrough was the ability to send messages to other machines on the Internet, using the @ sign to designate the receiving machine.

An email message has always been nothing more than a simple text message - a piece of text sent to a recipient. In the beginning and even today, email messages tend to be short pieces of text, although the ability to add attachments (e.g., pictures, documents) now makes many email messages quite long. Even with attachments, however, email messages continue to be text messages.

Email made up 75% of network traffic soon after the introduction of the internet.

What makes up an email?

An email message is constructed like a letter you'd send through the postal service: a message enclosed in an envelope.

The email envelope header is analogous to the envelope of a hardcopy letter, but some of the information that is ordinarily present on a hardcopy envelope is contained in the message header instead of the envelope header. This header also contains information that is not usually found on a real-world envelope, but is essential to email delivery and troubleshooting.

The envelope header is usually hidden when you view an email, and the message header is usually visible. Together, these two headers are called the full header.

Figure 1: The inbox contains information from the header, including sender, subject, date, and size

Message header fields

The content of these fields differs only slightly from regular mail, because the 'From:', 'To:' and 'Cc:', fields in an email identify the sender and intended recipients by email address.

The Date field - The message header's 'Date:' field is applied by the originating sender's MUA, so it is only as accurate as the clock on the sender's computer.

The Subject field - The 'Subject:' field isn't used in regular mail except in formal business letters where its closest analogy is the 'Re:' line.

The Return-Path - Email contains more detailed information about its delivery process than the single postmark of regular mail.

The 'Return-Path' is often referred to as the envelope sender, and this is the address that mailing lists use to determine who sent a message. The 'Return-Path' is also the address to which bounces are sent.

The Received field - A 'Received' field added by each MTA (mail transfer agent) in the email delivery chain as it accepts a message for transfer.

This information about the true identity of the sending MTA is valuable when troubleshooting issues with spam or malicious messages.

The Reply-To field - The envelope header also contains a 'Reply-To' address provided by the sender that the receiver can use to reply to the sender.

Figure 2: Information from the header is visible in the message, including sender, date, and subject

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