How Email Works

This topic provides a high-level description of how email works, including the mechanics and infrastructure of how email messages are delivered to their intended recipients.

Terminology

This section explains some of the common acronyms and terminology used when discussing email message and infrastructure.

Protocols: Protocols refer to an established, standardized set of rules that govern how systems interact. Within the context of email, the following protocols are utilized:

Simple Mail Transfer Protocol (SMTP): This protocol controls the processing of outbound email messages by defining the envelope information (recipient email address, return address, etc.) and the content of the message.
Post Office Protocol (POP3): This protocol controls the processing of inbound email messages. This protocol allows you to keep your inbound email messages on a mail server that your email client software can access and download to your local computer. Once downloaded, the message is deleted from the mail server.
Internet Mail Access Protocol (IMAP): Like POP3, this protocol controls the processing of inbound email messages, but it's more sophisticated than POP3. When the recipient downloads an email message from the mail server to his or her local computer, the message is not deleted from the server. This protocol allows the recipient to access the message from multiple devices and applications, such as a mail client application, a web interface, a smartphone, etc.

Domain Name Server (DNS): A public repository of internet domains and their corresponding IP addresses. The DNS is responsible for translating the human-readable email domain (such as "@cheetahdigital.com" for example) into an IP address that represents the physical location of that domain.
Mail Transfer Agent (MTA): Sends messages to the Internet Service Providers (ISPs) for delivery.

Email Address

A valid email address must follow the following syntax: username@domain.ext. During the data import process, Messaging will identify email addresses with an invalid syntax, and remove them from the import. For more information on the steps Messaging takes to process email addresses, please see Email Validation and Cleansing Options.

An email address consists of two critical pieces of information -- the user and the domain. The username portion consists of everything before the @ sign, and represents the individual mailbox, or recipient. The domain portion consists of everything after the @ sign, and indicates the Internet Service Provider (like Yahoo or Gmail), or an entity such as a business or a school or an organization. In postal mail terms, you could think of the domain as the ZIP code, and the username as the street address. The domain is intended to be human-readable representation of a physical IP address, and the username represents an individual account at that IP address.

Process

The following diagram depicts a typical process for sending an email message from one individual to another.

The sender writes the email message in his or her email client software, indicates the recipient's email address, and sends the message.
The message (and any attachments) is sent via SMTP to the outbound SMTP server.
The SMTP server looks up the recipient's domain with a Domain Name Server, in an attempt to identify the physical address for the recipient's email exchange server.
The SMTP server sends the message (and any attachments) via SMTP to the recipient's inbound mail exchange server, or Mail Transfer Agent (MTA).
The MTA puts the message in the recipient's inbox.
The recipient access the message, and downloads it to his or her local client software.

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