TapHow email works (2) Relays

What we saw in part 1 is the simple case with just a sending and receiving mail server, but quite often a number of intermediate hops are required to reach the ultimate destination. Using the “Received: ” headers in reverse order you can see what route a mail item took to get to you.

When email was invented, the networks weren’t as richly interconnected as they are now and consisted of small islands connected by narrow, unreliable pipes. So the technique to deliver a message was to forward it to another server which was a bit closer to the final destination. There was even a way you could suggest the route you wanted your mail to take. A system evolved where servers would discover the best routes by learned experience. This meant that servers became accustomed to accept any mail and forward it on as required.

In more recent times this custom was exploited as the servers didn’t care where the mail came from. Spammers arranged to send mail to random servers and specify obscure routes knowing that they would dutifully forward them on and their tracks would be hard to detect. This is called the “Open Relay Exploit.” To avoid this, servers began to be more selective and mostly they now only accept mail from their own clients or, alternatively, mail destined for places that they know about and are responsible for. They will no longer accept mail from anywhere to anywhere. In practice you will see from the headers that most mail gets moved around internally at the source end, makes a big leap to the destination, and then moves around a bit there before being stored.

An organisation called ORDB (Open Relay Database) tracks down the few remaining open relays and system managers can use this list to ban them from contact as their transmissions will be unreliable.

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