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.

Decorative image of the inbox in Microsoft Outlook, showing the sender (from), subject, received date, and size of messages in the inbox.

Image: The inbox contains information from the header, including sender, subject, date, and size.

Message header fields

The Date field

The Subject: field

The Return-Path

The Received field

The Reply-To field

Decorative image of an email message received in Outlook, showing information from the header (sender, recipient, subject, date) and the message body.

Image: Information from the header is visible in the message, including sender, date, and subject.


In this Course, you’ll further explore the web protocols that underpin the internet and the world wide web, and some of the applications they enable.

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