How email works: step-by-step, diagram

How email works: step-by-step, diagram

As simple as it is to use, an email relies on a more complicated set of operating procedures than that of the Web. For most users, its operation is transparent, which means that it is not necessary to understand how it works in order to be able to use it. The goal of this article is to help you to understand its basic principles, give you an idea of how to best configure your email clients, and inform you about the underlying mechanisms of spam.

How does email work?

The use of email is based on the use of electronic mailboxes. When an email is sent, the message is routed from server to server, to the recipient's email server. More specifically, the message is sent to the mail server tasked with transporting emails (called the MTA, for Mail Transport Agent) to the recipient's MTA. On the Internet, MTAs communicate using the protocol SMTP and are logically called SMTP servers (or sometimes outgoing mail servers).

The recipient's MTA then delivers the email to the incoming mail server (called the MDA, for Mail Delivery Agent) that stores the email as it waits for the user to accept it. There are two main protocols used for retrieving email on an MDA:

  • POP3: Post Office Protocol, the older of the two, which is used for retrieving emails and, in some instances, leaving a copy of them on the server.
  • IMAP: Internet Message Access Protocol, which coordinates the status of emails (read, deleted, moved) across multiple email clients. With IMAP, a copy of every message is saved on the server, so the synchronization task can be completed.

For this reason, incoming mail servers are called POP servers or IMAP servers, depending on which protocol is used: 

Schematic diagram of MTA-MDA-MUA

To use a real-world analogy, MTAs act as the post office (the sorting area and mail carrier) that handles message transportation, while MDAs act as mailboxes that store messages (as much as their volume will allow) until the recipients check the box. This means recipients don't need to be connected to receive an email.

To keep everyone from checking other users' emails, MDA is protected by a username called login and by a password.

Retrieving mail is done using a software program called an MUA (Mail User Agent). When the MUA is installed on the user's system, it is called an email client (such as Mozilla Thunderbird, Microsoft Outlook, Eudora Mail, Incredimail, or Lotus Notes). When it is a web interface used for interacting with the incoming mail server, it is called webmail.

What is an open relay?

By default, it is not necessary to authenticate oneself to send an email, so it is very easy to falsify one's address. For this reason, nearly all Internet service providers lock down their SMTP servers so that only their subscribers can use them, or more precisely, machines whose IP address belongs to the ISP's domain.

An open relay is called an open relay when an organization's email server is improperly configured and allows third-party users on any network to send emails. Spammers generally use open relays, as using them hides the true origins of their messages. As a result, many ISPs keep an up-to-date blacklist of open relays to keep subscribers from receiving messages from such servers.

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