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Email is one of the most essential things to our life. But do you actually know what happens when you click the “send” button, and how it's sent to your friends?

Hosted by: Hank Green

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Hank: Your email inbox is a battleground, where your parents and bosses and criminals and that place you bought a scarf from five years ago all compete for your attention. And even with texting and other electronic message systems becoming more and more common, email still reigns supreme.

Humans send ten times more emails than texts, adding up to more than two million emails sent every single second. But what actually happens to an email after you send it?

I mean, you hit “send”, and then magic happens in a minute later your friend has the message pop up in front of them, whether they’re across the room or across the world.

It might seem like a simple little thing, but think about it, you have no idea how this simple little thing happens and there is actually a lot of technology and systems involved. Some of those systems help get your email get to the right place, bouncing it from computer to computer and across networks. Other processes are designed to protect you and other users, running your email through sophisticated spam filters that all sit between your computer and your friend’s inbox, checking for things like spoofing and phishing.

And it all begins with the website or service that you log into to check and send emails in the first place. You might use a webmail service like Gmail or Yahoo, or a desktop email client like Outlook or Thunderbird. There are lots of different options.

And you already know that if you don’t fill in a recipient, which is just the “To” field, the email won’t go anywhere. But that’s not all it takes. For every email you send, there’s a big part of it that you probably never see, called the header. The header contains a kind of summary of the basic info about your email, like who it’s from and who it’s going to. That can be helpful in finding spammers and other criminals based on where their emails come from, but headers have another purpose, too: The header will tell you everything that’s happened to the email — where on the internet it came from, where it’s going, and all the places it stopped along the way.

And emails do bounce between a lot of computers on their journey. The first order of business involves figuring out how to find your friend among all of the billions of computers on the internet. So, once you hit Send, you say goodbye to your email — but when you say goodbye, your email client says “hello.” It actually, literally says “hello”.

Your client connects to its Simple Mail Transfer Protocol, or SMTP server. This is basically a whole separate computer in charge of sending your email to the right place, and it’s the same system that we’ve been using system beginning of the internet and no one owns or controls it, everybody gets to use it equally, pretty amazing if that actually works at all. But your email client has to get the SMTP server’s attention first before it can release your email.

And it does that with a nice, friendly “HELO” command, H-E-L-O, which is how it identifies itself to the SMTP server and lets it know that there’s an email that needs sending. After that, the SMTP server grabs your email and brings it to your client’s Mail Transfer Agent, or MTA. Yes, there are going to be lots of initialisms involved here.

So, the SMTP server acts a lot like a mail carrier who picks up a package or letter at your house and drops it off at your local post office -- with the MTA acting like the post office. The problem is — and I’m not sure if you’ve noticed here— the internet is enormous. It wouldn’t be practical or efficient or possible for your MTA to know how to send an email to every possible email address on the internet.

So your MTA doesn’t know what wires or cables to send an email down to get it to your friend, and it doesn’t need to. All it needs to know is how to send your email in the right direction. It’s like how the guy at my post office doesn’t like take my letter and walk up to the door, and then drive to Washington, and give it to my friend.

Instead, they’ll forward the letter through a bunch of different post offices until it gets to the right place. So first, the MTA asks what’s known as a Domain Name Server, or DNS, which is like a big set of tables that tell the MTA where to find your friend’s inbox. Then, the MTA uses the DNS to map out a path, and sends your email along to a different MTA that is closer to your friend’s inbox.

Then this new MTA just repeats the process. It starts by checking where the email needs to go, if it knows how to get there, great! Otherwise, it checks a DNS and sends the email to an MTA server that’s even closer to the right place, which then repeats the process.

Eventually, your email lands at an MTA that belongs to your friend’s email client, which knows exactly what to do with it. That would be my Washington’s local post office. The MTA calls on the SMTP server one more time to send your email to the client’s servers -- the big, central computers that store all the emails that come into that client.

But before your email lands at your friend’s inbox, it has to pass the spam filter. Spam is unsolicited bulk email: unsolicited because you didn’t ask for it, and bulk because it can be sent to hundreds or thousands or even millions of people at once. These days, a little over 50% of the emails sent are spam.

Sometimes, spam is from legitimate companies that just want you to buy something. But a lot of the time is more than that. Spam emails link to websites or have attachments with viruses, or they can try to scam people out of money with a fake story and a bank account to send money to.

You probably know that you have a spam folder, where you can check to see if you missed any emails from Nigerian princes. But there are a ton of emails that never even make it as far as the spam folder — they’re filtered out and deleted before they get to you. Mail clients are understandably pretty tight-lipped about exactly what they do to filter spam. Because the more spammers know about the process, the easier it is for them to beat the filters. But there are at least a couple basic things that pretty much everyone looks at first.

They’ll check if the sender has an email address that seems legit, if links in the email go to where they say they do, and if the email’s text seems to make some basic sense. Spammers are always searching for holes in the system, which is why you’ll sometimes see spam that seems sort of pointless. It doesn’t ask for money, it doesn’t have attachments, it doesn’t any links at all. It just has some weird, usually misspelled nonsense, like “Hi, I”M Bobb. Itwas nIce// t0 meet you[sparkle emoji] you. Bye..[Heart emoji].” The goal of these emails isn’t to trick you. It’s to see if that weird email tricks your client’s filter.

Because if a message like that gets through a hole, then spammers know, next time, they can send something more malicious through the same hole. A lot of email clients also have blacklists: Lists of email addresses or locations that send so much spam that they’re automatically blocked. No one tells spammers if they’re on a blacklist, of course, but they figure it out soon enough, since none of their emails are getting through.

And some spammers try to get around these blacklists by changing the information in their emails’ headers. Every MTA along an email’s path adds a line to the header saying where it is and when it received the email, so spammers try to change these lines after the fact. Or they can try to make it look like the email came from a different address or location than it actually did.

This is called spoofing, and spammers use it to make it seem like their emails are from somewhere more legitimate -- or, at least, somewhere that is not blacklisted. Spoofing is often used in phishing attacks, where the attackers pretend to be from a legitimate company so you’ll give them your personal information. You can see how spoofing would be useful for this: if an email looks like it’s coming from your bank and says that your account has been compromised, you’re much more likely to click on the link in the email to login to your account.

When you click the link, this kind of phishing attack might take you to a website that looks exactly like your bank’s homepage. You enter your username and password... and now the attackers have your log-in information to your bank account. Which is not what you want. That’s why companies often say that they’ll never ask for your personal information or password in an email — this way if you get an email that seems to be from your bank, but asks for your password, you know that is definitely a scam.

It’s disturbingly easy to spoof by changing an email’s header, but there are a few protections in place to help weed out spoofers. Lots of legitimate email services use an authentication scheme, where MTAs can check to see if the server sending the email is actually tied to the email address it says the email is coming from.

If an email looks like it’s been spoofed, these email services will warn you when you click on the email. So, filtering out clever spammers without blocking legitimate emails is a constant battle for every email client. But you’re no spammer, and your friend’s email client knows that.

So your email gets marked for their inbox and sits on the servers until your friend checks their email. They open it up and read what you wrote. And when they reply, if they were reply please reply to my e-mails, the whole bouncy process starts over in reverse.

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