How Electronic Mail Works For Laymen.

[Michael St. Laurent]

Jason, that's one of the funniest technical papers I've ever read!  I almost
fell out of my chair.

Should this go into the FAQ folks?

Jason Balicki <mailto:kodak at FRONTIERHOMEMORTGAGE.COM> wrote:
> This appears to be desperately needed:
>
> How Electronic Mail Works For Laymen
>               -Or-
> How I learned to stop worrying and love acronyms.
>
> Every time you type a message, small multi-headed trailer-
> park demons (or SMTP's for short) take that message and run
> inside the network cables (or phone lines, but they're
> narrower, that's why they're slower) to the destination
> where it's handed to the incorrigible mail altered-state
> postal demons (or IMAPD) to then be handed out by
> the positronic otter people (or POP) to the users
> (aka idiots.)  Incidentally, this is also why
> telephone lines sag from pole to pole.
>
> If MailScanner is running on your system, then
> there are a few more steps involved, but mostly
> it's just small portly aristocratic men making
> sure that you don't have the word "penis" in
> your post.
>
> Not to be confused with the Kraft Foods Post[tm]
> brand cereal products.  That would be gross.
>
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> Still here?
>
>
> Ok, here's how it really works, but keep it secret
> so us admin/priests still have jobs:
>
> [note to the pedants:  yes, this is simplified --
> it's meant to be.  A lot of documentation out there
> seems to be needlessly complicated, especially in
> the context of someone just starting out.  I
> realize it's fun to abuse our PFY's, but they've
> got to know something if we're going to be able
> to sneak out to a pub at 10AM.  Further note:
> I wrote this in a hurry.]
>
> There are three parts to a mail system:
>
> Mail User Agent (MUA).
> Mail Transfer Agent (MTA).
> Mail Delivery Agent (MDA).
>
> The MUA is what the user uses to create (and usually read) mail.
> It's what's usually referred to as the "client".  This can be
> Outlook, Outlook Express, Evolution, a web browser connecting
> to a web-based email system (Hotmail) etc.
>
> The MTA is the meat and potatoes (or beans and franks, if you
> prefer).  It, surprisingly enough, transfers mail; usually
> between an MUA and a MDA.  Some packages that do this are
> Sendmail, Exim, and Postfix.  This list isn't exhaustive,
> don't get your panties in a bunch if I don't name your
> favorite.
>
> The MDA takes the message from the MTA and delivers
> it to the user.  This is some murky water right here,
> because, technically, all MTA's have MDA components,
> but usually when someone is talking about a MDA they're
> talking about the software at the end of the chain
> on the server, the part that hands the message out,
> like a pop3 server or an imap server.  Some big packages
> seem to be both an MTA and an MDA (like Exchange) but
> internally they are separate components.
>
> Here is (a very simplified) chain of events where we
> follow a message from a sender to the destination.
>
> bob at fromhere.com is sending a message to tom at tohere.com:
>
> 1) Outlook [MUA] (Bob) (Sender)
>       |
> 2) Sendmail [MTA] (mail.fromhere.com)
>       |
> 3) Sendmail [MTA] (mail.tohere.com)
>       |
> 4) imapd [MDA] (mail.tohere.com)
>       |
> 5) Outlook [MUA] (Tom) (Recipient)
>
> 1) Bob fires up Outlook, types his message to Tom
>
> 2) outlook connects to the "outgoing" mail server
> in its configuration, in this case let's say it's
> mail.fromhere.com.  mail.fromhere.com sees that
> the mail is to go to mail.tohere.com so it in
> turn connects to
>
> 3) mail.tohere.com.  mail.tohere.com realizes
> that the mail is destined for local delivery,
> so it puts it in toms spool that
>
> 4) imap knows about.  When Tom fires up
> his client and it connects to the imap
> server that lives on mail.tohere.com
> it hands the message over to
>
> 5) Toms Outlook, that he then reads:
>
> "Hey, Tom, do you ever get that unfresh feeling?
>
> Love,
> Bob."
>
> So, now you're probably asking yourself
> "why the hell am I reading this garbage?"
>
> And I'd have to answer "because you want
> to know how MailScanner fits into this, punk!"
>
> And then I'd be like "You been shown."
>
> and then you'd be all "Oh, yeah?  It's ON!"
>
> and I'd be like "Bring it, bitch!"
>
> and then I'm all:
>
> To simplify things we're going to just look at
> the receiving end of the mail conversation
> I described above, so Bob's sent his message
> and it's now coming into mail.tohere.com:
>
> 1) Sendmail incoming queue mail.tohere.com (MTA)
>        |
> 2) Mailscanner on mail.tohere.com (sort-of-MTA)
>        |
> 3) Still Mailscanner: Checks for viruses and spam
> (usually using 3rd party utilities)
>        |
> 4) Sendmail delivery queue (mail.tohere.com) (MTA)
>        |
> 5) imapd (mail.tohere.com) (MDA)
>        |
> 6) Outlook (Tom) (MUA)
>
> 1) sendmail takes the message and immediately puts
> it in a queue, in this example, that's it.
>
> 2) MailScanner picks it up from the queue and
>
> 3) beats the crap out of it, if it survives it's
> put in another queue that
>
> 4) Sendmail picks up and delivers to the users
> spool that
>
> 5) imap knows about and delivers to the user
> when
>
> 6) he fires up Outlook.
>
> [note: there's a really cool diagram available at
> http://www2.essex.ac.uk/cs/services/email/anti-spam/mailscanner.html
> that covers this part in more detail.  If you're
> into that sort of thing.]
>
> As you learn more about the mail process you'll learn
> that 1) most of what I've written here is incredibly
> simplified and 2) doing this crap day in and day out
> is a sure-fire way to end up in the nuthouse.
>
> There is much more to all of this, and explaining
> it might (and I'm guessing here) almost fill up a book.
> Or two hundred.
>
> If you're seriously interested you need to get your
> grubby lil' paws on a box and install all this software
> on it.  Play around.  Get a couple of boxes and try
> getting them to send mail to each other.  Get yourself
> a dyndns account and set up a test mail server at
> home.  Play around and learn.  You could go take
> classes, but that costs money and the reason you're
> into this is because you think you might be able to
> get a job doing it because right now you work at
> a fast food joint and don't make jack and can't
> afford the classes.
>
> Well, that's why I said to use the free stuff.  Then
> you can be a highly paid and mentally stable systems
> administrator like me!  Twitch.
>
> --J(K)
>
> PS:  Slow day, what can I say.  If you don't like it,
> it's not my fault you read it all the way to the
> end.



--
Michael St. Laurent
Hartwell Corporation