How Email Is Sent and Received.

Email is transmitted as plain text across networks around the world using the SMTP protocol (Simple Mail Transfer Protocol). As the name implies, the protocol itself is fairly basic, and it has been extended to add further authentication and error reporting/messaging to satisfy the growing demands of modern email.

Mail transfer agents, or MTAs, work in the background transferring email from server to server allowing emails to be sent all over the world.

SMTP allows each computer that the email passes through to forward it in the right direction to the final destination. When you consider the millions of email servers across the world, you have to marvel at how simple it all seems.

Here is a simplified example of how email is successfully processed and sent to its destination:

1. phil@sender.com composes and sends an email message to matt@receiver.co.uk.

2. The MTA at sender.com receives Phil's email message from Outlook, Outlook Express or whatever email program Phil uses and queues it for delivery behind any other messages that are also waiting to go out.

3. The MTA at sender.com contacts the MTA at receiver.co.uk. After receiver.co.uk acknowledges the connection, the MTA at sender.com sends the mail message. After receiver.co.uk accepts and acknowledges receipt of the message, the connection is closed.

IMPORTANT FACT
An MTA will not send an email until the destination MTA signals it is ready and willing to accept the email and it will not be marked as sent until the receiving MTA acknowledges that it has sucessfully received the email !

4. The MTA at receiver.co.uk places the mail message into Matt's incoming mailbox; Matt is notified that he has new mail the next time he logs on.

Of course, several things can go wrong during this process. Here are a few examples:

What if Matt does not exist at receiver.co.uk? In this case, the MTA at receiver.co.uk will reject the email and notify the MTA at sender.com of what the problem is. The MTA at sender.com will then generate an email message and send it to phil@sender.com, informing him that no Matt exists at receiver.co.uk (or perhaps just silently discard the message and give the sender no indication of the problem, depending on how the email server is configured).

What happens if receiver.co.uk doesn't respond to sender.com's connection attempts? (Perhaps the server is down for maintenance.) The MTA at sender.com notifies the sender that the initial delivery attempt has failed. Further attempts will be made at intervals decided by the server administrator until the deadline is reached, and the sender will be notified that the mail is undeliverable.