Email on the Internet

This page aims to give you a brief overview of how email moves on the Internet. The following diagram summarises the sequence of events.

Summary

Often the Sending Mail Server and the Receiving Mail Server are run by the same people (eg. your ISP). However, this need not be the case. You can use one machine/organisation to send email, and another one to use as a mailbox to receive and store email.

Guest accounts may not use FastMail as their Sending Mail Server (eg SMTP Server) when sending emails from home. Generally this is not a problem as your ISP will give you an SMTP server you can use.

Sending Email

  1. From your home computer, a connection is made to the Sending Mail Server (otherwise known as the SMTP Server)
  2. The email is sent via the SMTP protocol to the Sending Mail Server
  3. The Sending Mail Server holds the email and places it in a queue
  4. Based on who the email was addressed to, the Sending Mail Server connects to the destination machine, the Remote User Receiving Mail Server, and tries to send on the email again using the SMTP protocol
  5. Depending on if the machine is overloaded, not available, network connection problems, etc, the transmission may fail. In this case, the Sending Mail Server usually leaves the email in the queue and tries again at a later time. Usually it will try again for between 1 hour and 7 days to resend the message. If it doesn't succeed, an error response is returned to the user who originally sent the message
  6. Once the email arrives at the Remote User Receiving Mail Server, it's up to that server to deliver it to a users mailbox, forward it elsewhere, etc

Receiving Email

  1. The Remote User Sending Mail Server connects to the Receiving Mail Server and sends the message using SMTP
  2. The Receiving Mail Server works out what user the message is being directed to, and delivers it to that users mailbox
  3. When the user is online/ready, they connect from their home machine to the Receiving Mail Server server via the POP (sometimes called POP3) or IMAP (sometimes called IMAP4, IMAP4rev1, etc) protocol
  4. They then retrieve the email from their mailbox on the Receiving Mail Server. Depending on the protocol and settings, the email may be deleted from the mailbox on the server, or it may be left on the server