eli5: How exactly are emails sent, like the actual process of the data getting sent to another device that’s for example on another continent?
In: Technology
Internet communications are generally governed by the Open Systems Interconnection (OSI) Model. The OSI model works as a set of different protocols that get layered on top of each other, adding additional information as you go up the layers.
Layer 1 is the Physical layer. This is the (until the creation of wi-fi) actual wire that’s connecting two devices, where data is sent as ones and zeros at a certain rate of speed, with certain error correcting mechanisms.
Layer 2 is the Datalink layer. This covers communication between a group of devices on the same physical network, and includes ways for each device to uniquely identify itself, and to send information to one specific device, or broadcast to everyone on a network. If you’ve ever heard of MAC addresses, this is the layer that those come into play
Layer 3 is the Network layer. This is the heavy lifting layer that really forms the Internet, and is here the ideas of routing traffic between networks and TCP/IP addresses get involved.
Layers 4-7 we’ll just gloss over for now as they don’t come into play much for our purposes.
Emails are generally sent over an Internet protocol called Simple Mail Transfer Protocol, or one of its more secure descendants. This handles connections to a mail server for sending email out, and connections between mail servers for delivery.
Now comes the metaphor. Think of your computer/mail client program as your house, and the local post office as the mail server. You mail out a letter by going to the post office with a message, you put a return address on it and a delivery address on it. and the post office takes it. That’s what SMTP does for you. It talks to the server, inputs a destination e-mail address, a “from” e-mail address and a message.
Now the post office works to deliver your letter. Based on the destination ZIP code, it knows to get the letter to the post office for that particular zip code. It then gets fed into the whole shipping and logistics operation of the mail to get to that destination. Similarly, the mail server looks at the domain for the destination e-mail (the part after the @), and uses another protocol called Domain Name Service (DNS) to look up what TCP/IP address the mail server for that domain is. Then it uses TCP/IP to transfer the message across the internet to the destination mail server wherever that may be.
When the destination post office gets the letter, it stores it in your PO box (to keep the metaphor, it doesn’t deliver the mail). The email server similarly keeps the message in under your username.
When you go to the post office to check your PO Box, you use your key, then check to see if there are any letters in your mailbox, take them out, and bring them home to open them.
In the digital world, your email client program connects to the server, gives its username and password (key) checks the number of new messages in the mailbox and downloads them.
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