January 2, 2008
Have you ever wondered what happens when you press the send button for an email? How does it get from your machine to the recipients inbox by giving a simple address? What is meant when the message “bounces” or by the terms “email headers” or “return address”? Everything becomes a little clearer if we look at a simple analogy between sending a letter by courier and sending an email.
You first write the letter and then place it in an envelope. On the envelope you write the address of the recipient. Now because you want this letter to arrive quickly you decide to use a courier. You give the letter to the courier who then puts your envelope in a special courier envelope which contains handling information such as the senders original address.
When you send an email the process is very similar. You write your email, specify the recipient address and press the send button. The email is then sent to the local mail server (SMTP server) which adds its own envelope to the message which includes a return address and puts the message in a queue for processing.
The envelope is then taken to the local depot where the courier then works out the best way to deliver the message. They look at the address and deciding if it can be delivered by the local depot or whether it needs to be sent to another depot for delivery. If another depot is required then the logistics department works out a route between courier depots to the letter’s final destination.
Once at the head of the queue the server looks at the address of the recipient to see if the message can be delivered locally. Local means that the mail server is responsible for delivery for that domain. For example, if you are sending an internal email to a colleague in the same office it would be pointless for the email to travel around the internet when the address is local.
If it can then it attempts local delivery otherwise a Domain Name Server (DNS) lookup of the recipients address is performed and the IP address is passed to the router which works out the best route between email servers on the internet.
The letter then travels between the different depots, with each depot adding its own handling information to the courier envelope. At any point on its travels a number of things could happen to the letter to delay its delivery. It might miss a connecting flight to another depot or a depot maybe temporarily closed.
In either case delivery will be re-attempted until eventually the letter arrives at the destination depot. The mail server then connects to the receiving mail server using Simple Mail Transfer Protocol (SMTP).
The receiving server then adds its own “email headers” to the message envelope and looks to see if the message can be delivered locally. If not the next route is calculated and a connection to the next SMTP server initiated. A message maybe passed to a number of SMTP servers before arriving at its intended destination.
Final local delivery is then attempted by the courier. If the courier cannot find the address that is written on the envelope then the letter is returned to the sender. For example a depot may be responsible for delivery in Manchester but might not be able to find a street named “Worsley Street”. In this instance the depot will return the letter to the sender using the sender address on the original courier’s envelope.
Once local delivery is attempted the mail server checks to see if the recipient’s address exists. For example, a server maybe responsible for handling delivery of the domain “achme.com” but when it attempts delivery of a message to [email protected] it discovers that bill doesn’t exist. In this case the SMTP server will return the message to the sender by looking at the return address on the message envelope. This is known as a hard bounce.
If the courier finds the correct address for delivery but nobody answers the door, or for our analogy, the letter box is full, then he will take the letter back to the depot with him and attempt redelivery the next day. This will happen maybe 3-5 times. If after this time the letter can still not be delivered then it will be returned to the sender.
If the address exists then the server will try to deliver the email to the recipient’s inbox. If the inbox is full or the server cannot deliver the message to the inbox then it will stay on the server’s delivery queue and delivery will be attempt again in 24hrs. If after 3-5 times the message can still not be delivered then the message will be returned to the sender. This is known as a soft bounce.
The recipient picks up their mail from his letter box and opens the letter you sent. The recipient retrieves their email from their inbox and opens the message.
Posted by Simon Hill
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