A friend of mine asked me how it is possible that she pushes buttons on her keyboard and mouse, and in an instant her peer reads the text she had in her mind. This is a step-by-step introduction of what happens in-between.
From your mind to your computer
When you decide to write an e-mail to an acquaintance of yours, you open up your mailing software (this document doesn’t cover using mail applications you access through your browsers, just plain old Thunderbird, Outlook or similar programs. However, it gets the same after the mail left your computer), and press the “New Mail” button. What happens during this process is not covered in this article, but feel free to ask me in a comment! Now that you have your Mail User Agent (MUA) up and running, you begin typing.
When you press a button on your keyboard or mouse, a bunch of bits gets through the wire (or through air, if you went wireless) and get into your computer. I guess you learned about Morse during school; imagine two Morse operators, one in your keyboard/mouse, and one in your computer. Whenever you press a key, that tiny creature sends a series of short and long beeps (called 0 or 1 bits, respectively) to the operator in your computer (fun fact: have you ever seen someone typing at an amazing speed of 5 key presses per second? Now imagine that whenever that guy presses a key on their keyboard, that tiny little Morse operator pressing his button 16 times for each key press, with perfect timing so that the receiving operator can decide if that was a short or long beep.)
Now that the code got to the operator inside the machine, it’s up to him to decode it. The funny thing about keyboards and computers is that the computer doesn’t receive the message “Letter Q was pressed”, but instead “The second button on the second row was pressed” (a number called scan code). At this time the operator decodes this information (in this example it is most likely this Morse code: ···-···· -··-····) and checks one of his tables titled “Current Keyboard Layout.” It says this specific key corresponds to letter ‘Q’, so it forwards this information (I mean the letter; after this step your computer doesn’t care which plastic slab you hit, just the letter ‘Q’) to your MUA, inserts it into the mail in its memory, then displaying it happily (more about this step later).
When you finish your letter you press the send button of your MUA. First it converts all the pretty letters and pictures to something a computer can understand (yes, those Morse codes, or more precisely, zeros and ones, again). Then it adds loads of meta data, like your name and e-mail address, the current date and time including the time zone and pass it to the sending parts of the MUA so the next step can begin.
IP addresses, DNS and protocols
The Internet is a huge amount of computers connected with each other, all of them having at least one address called IP address that looks something like this: 123.234.112.221. These are four numbers between 0 and 255 inclusive, separated by dots. This makes it possible to have 4,294,967,296 computers. With the rules of address assignment added, this is actually reduced to 3,702,258,432; a huge number, still, but it is not enough, as in the era of the Internet of Things everything is interconnected, up to and possibly including your toaster. Thus, we are slowly transitioning to a new addressing scheme that looks like this: 1234:5678:90ab:dead:beef:9876:5432:1234. This gives an enormous amount of 340,282,366,920,938,463,463,374,607,431,768,211,456 addresses, with only 4,325,185,976,917,036,918,000,125,705,034,137,602 of them being reserved, which gives us only a petty 335,957,180,944,021,426,545,374,481,726,734,073,854 available.
Imagine a large city with that many buildings, all of them having only a number: their IP address. No street names, no company names, no nothing. But people tend to be bad at memorizing numbers, so they started to give these buildings names. For example there is a house with the number 216.58.209.165, but between each other, people call it gmail.com. Much better, isn’t it? Unfortunately, when computers talk, they only understand numbers so we have to provide them just that.
As remembering this huge number of addresses is a bit inconvenient, we created Domain Name Service, or DNS for short. A “domain name” usually (but not always) consist of two strings of letters, separated by dots (e.g. polonkai.eu, gmail.com, my-very-long-domain.co.uk, etc.), and a hostname is a domain name occasionally prefixed with something (e.g. www.gmail.com, my-server.my-very-long-domain.co.uk, etc.) One of the main jobs of DNS is to keep record of hostname/address pairs. When you enter gmail.com (which happens to be both a domain name and a hostname) in your browser’s address bar, your computer asks the DNS service if it knows the actual address of the building that people call gmail.com. If it does, it will happily tell your computer the number of that building.
Another DNS job is to store some meta data about these domain names. For such meta data there are record types, one of these types being the Mail eXchanger, or MX. This record of a domain tells the world who is handling incoming mails for the specified domain. For gmail.com this is gmail-smtp-in.l.google.com (among others; there can be multiple records of the same type, in which case they usually have priorities, too.)
One more rule: when two computers talk to each other they use so called protocols. These protocols define a set of rules on how they should communicate; this includes message formatting, special code words and such.
From your computer to the mail server
Your MUA has two settings called SMTP server address SMTP port number (see about that later). SMTP stands for Simple Mail Transfer Protocol, and defines the rules on how your MUA, or another mail handling computer should communicate with a mail handling computer when sending mail. Most probably your Internet Service Provider gave you an SMTP server name, like smtp.aol.com and a port number like 587.
When you hit that send button of yours, your computer will check with the DNS service for the address of the SMTP server, which, for smtp.aol.com, is 64.12.88.133. The computer puts this name/address pair into its memory, so it doesn’t have to ask the DNS again (this technique is called caching and is widely used wherever time consuming operations happen).
Then it will send your message to the given port number of this newly fetched address. If you imagined computers as office buildings, you can imagine port numbers as departments and there can be 65535 of them in one building. The port number of SMTP is usually 25, 465 or 587 depending on many things we don’t cover here. Your MUA prepares your letter, adding your e-mail address and the recipients’, together with other information that may be useful for transferring your mail. It then puts this well formatted message in an envelope and writes “to building 64.12.88.133, dept. 587”, and puts it on the wire so it gets there (if the wire is broken, the building does not exist or there is no such department, you will get an error message from your MUA). Your address and the recipient’s address are inside the envelope; other than the MUA, your own computer is not concerned about it.
The mailing department (or instead lets call it the Mail Transfer Agent, A.K.A. MTA) now opens this envelope and reads the letter. All of it, letter by letter, checking if your MUA formatted it well. More than probably it also runs your message through several filters to decide if you are a bad guy sending some unwanted letter (also known as spam), but most importantly it fetches the recipients address. It is possible, e.g. when you send an e-mail within the same organization, that the recipient’s address is handled by this very same computer. In this case the MTA puts the mail to the recipient’s mailbox and the next step is skipped.
From one server to another
Naturally, it is possible to send an e-mail from one company to another, so these MTAs don’t just wait for e-mails from you, but also communicate with each other. When you send a letter from your example@aol.com address to me at gergely@polonkai.eu, this is what happens.
In this case, the MTA that initially received the e-mail from you (which happened to be your Internet Service Provider’s SMTP server) turns to the DNS again. It will ask for the MX record of the domain name specified by the e-mail address, (the part after the @ character, in my case, polonkai.eu), because the server mentioned that must be contacted, so they can deliver your mail for me. My domain is configured so its primary MX record is aspmx.l.google.com and the secondary is alt1.aspmx.l.google.com (and 5 more. Google likes to play it safe.) The MTA then gets the first server name, asks the DNS for its address, and tries to send a message to the 173.194.67.27 (the address of aspmx.l.google.com), same department. But unlike your MUA, MTAs don’t have a pre-defined port number for other MTAs (although there can be exceptions). Instead, they use well-known port numbers, 465 and 25. If the MTA on that server cannot be contacted for any reason, it tries the next one on the list of MX records. If none of the servers can be contacted, it will retry based on a set of rules defined by the administrators, which usually means it will retry after 1, 4, 24 and 48 hours. If there is still no answer after that many attempts, you will get an error message back, in the form of an e-mail sent directly by the SMTP server.
Once the other MTA could be contacted, your message is sent there. The original envelope you used is discarded, and a new one is used with the address and dept. number (port) of the receiving MTA. Also, your message gets altered a little bit, as most MTAs are kind enough (ie. not sneaky) to add a clause to your message stating “the MTA at <organization> has checked and forwarded this message.”
It is possible, though not likely, that your message gets through more than two MTAs (one at your ISP and one at the receiver’s) before arriving to its destination. At the end, an MTA will say that “OK, this recipient address is handled by me”, your message stops and stays there, put in your peer’s mailbox.
The mailbox
Now that the MTA has passed your mail to the mailbox team (I call it a team instead of department because the tasks described here are usually handled by the MTA, too), it reads it. (Pesky little guys these mail handling departments, aren’t they?) If the mailbox has some filtering rules, like “if XY sends me a letter, mark it as important” or “if the letter has a specific word in its subject, put it in the XY folder”, it executes them, but the main point is to land the message in the actual post box of the recipient.
From the post box to the recipients computer
When the recipient opens their MUA, it will look to a setting usually called “Incoming mail server”. Just like the SMTP server, it has a name and port number, along with a server type. This type can vary from provider to provider, and is usually one of POP3 (pretty old protocol, doesn’t even support folders on its own), IMAP (a newer one, with folders and message flags like “important”), MAPI (a dialect of IMAP, created by Microsoft as far as I know), or plain old mbox files on the receiving computer (this last option is pretty rare nowadays, so I don’t cover this option. Also, if you use these, you most probably don’t really need this article to understand how these things work.) This latter setting defines the protocol, telling your MUA how to “speak” to the post box.
So your MUA turns to the DNS once more to get the address of your incoming mail server and contacts it, using the protocol set by the server type. At the end, the recipients computer will receive a bunch of envelopes including the one that contains your message. The MUA opens them one by one and reads them, making a list ordered by their sender or subject, or the date of sending.
From the recipient’s comupter to their eyes
When the recipient then clicks on one of these mails, the MUA will fetch all the relevant bits like the sender, the subject line, the date of sending and the contents itself and sends it to the “printing” department (I use quotes as they don’t really print your mail on paper, they just convert it to a nice image so the recipient can see it. This is sometimes referred to as a rendering engine). Based on a bunch of rules they pretty-print it and send it to your display as a new series of Morse codes. Your display then decides how it will present it to the user: draw the pretty pictures if it is a computer screen, or just raise and lower some hard dots that represents letters on a Braille terminal.