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If I use some desktop-based email program like Mozilla Thunderbird on my own individual computer and someone on one of the other gazillion computers in the world writes me an email, how does the email find its way to me?

Furthermore, if the email program is on a local computer and not centralized, what prevents two or more people from making accounts with the same exact name?

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The programs are clients to a mail server. The web based email clients are also just that, clients. The mail server itself is another service. Either way these clients communicate with the actual mail server when you check your mail or send a message. The mail sent to you is mapped to the particular email server, and not your client software. That is why you will only have one instance of a username for any given domain.

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  • I see. Your sentence stating that emails to me go to the email server and not directly to the client software makes it clear. Thank you.
    – mring
    May 5, 2010 at 0:17
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Here you have decent explanation how email works with nice pictures :)

To answer you questions:

  1. Email finds a way as any other network packet - via an ip address - there's not difference in that.

  2. Your SMTP server needs to be able to find MX records of domain of your recipient - in this case it needs to be centralized. Check this out - open command prompt and enter:

    nslookup [enter]
    set type=mx [enter]

    gmail.com

This is how you find smtp servers :) So if send mail to [email protected] it knows where to go. Further more - if I have my own smtp server I can send you email signed by any address like [email protected] (it's just a piece of text in my e-mail) - but your provider smtp probably won't receive this mail, or throw it into spam cause it won't trust this server.

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  • Thanks, I didn't know about nslookup. That seems like a pretty useful tool. I tried to send an email to [email protected] instead of [email protected] where 209.85.222.93 was one of the ip addresses nslookup seemed to have given for gmail.com, but it didn't work.
    – mring
    May 5, 2010 at 0:39
  • It failed because gmail couldn't find your account. Gmail has your account configured as [email protected] not you@209... If I send a messenger to your home I will give him your street name and apartment number not gps coordinates, even though he will probably use gps to get there :)
    – kyrisu
    May 5, 2010 at 11:05
  • It failed before that. Email addrsses are assumed to always contain domain names; if you use a literal IP address, you have to use it like foo@[209.85.222.93], and this syntax is disabled in more and more servers due to abuse.
    – u1686_grawity
    Sep 15, 2011 at 17:37
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Your email program does a lot of things for you behind the scenes. This includes setting your username and password, etc. You could learn about the process by accessing your email with telnet. Also, some clients, like Thunderbird, allow you to view the email source. You might be surprised to know the information stored there.

There are several reasons why your attempt to use the IP address did not work. Perhaps it expects the mailbox domain to be spelled out, or perhaps you didn't set your password. If you really want to learn about email, download sendmail and try it out on your own computer.

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