How does the email system work? SMTP, POP, IMAP explained

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This page gives you the foundations of the email system.

At the end, you should be able to understand why when you configure your email client, you need to set up:

If you want to deliver an email to [email protected], you need to:

Create the email message
Lookup the MX records for the domain datacadamia.com to find the final SMTP server known as the MDA (Mail Delivery Agent)
Choose the first server by priority
Connect to a submission port (say 25) and use the SMTP protocol to transmit the message
The server stores it in a inbox (From a processing point of view, the server may also apply other filters such as forward it, store it on disk, send it to a rest API, ..)

If you do that from your home, you may encounter the following problems:

The port 25 is blocked by your Internet Service Provider (ISP)
Your email will be rejected because you are not authentified meaning that:
- Your email is not signed with DKIM
- Or your public IP is not in the SPF record
Your public IP may have a bad reputation and your email will land into the spam folder

That's why most of the time people send their emails from a fixed SMTP server known as a MTA (Message Transfer Agent)

From your desktop client configuration, Emails are:

sends to the MTA (Message Transfer Agent) SMTP server
that sends them to the final destination known as the MDA (Mail Delivery Agent) SMTP server
that stores them in the mailbox.

Your emails are read from the inbox via a Pop/Imap server and transmitted to your desktop client.