In the last two posts, we’ve talked about how email is handled in DNS. It’s an important first step, you can’t send email if you don’t know where it’s going to go. Now we’re going to talk about two parts of the email system called the MUA and MTA.

The MUA is the Mail User Agent. How’s that for a boring term? It means that program you (the user) are using to send and read email. That page in your web browser, that application on your phone, whatever you’re using, that’s called a MUA. I’ll list a few examples:

• elm (REALLY OLD)
• pine (Not as old as elm, but OLD)
• outlook (Microsoft’s flagship!)
• your web browser of choice
• thunderbird (OLD)
• mail (aka that app on your Mac)

That’s just a few of the programs that can send and read email. Your system doesn’t actually receive it though, that’s the job of the MTA. Your MUA doesn’t do the real job of sending it either, it hands that off to the MTA as well.

So what’s an MTA? An MTA is a Mail Transfer Agent. It’s the computer program that actually sends and receives your email. You don’t see it work, it’s like when you send your email you drop it into a box:

A box

the MTA picks it up and sends it along, like a postman:

Postal Van

And it lands in the other person’s Inbox:

Inbox

Of course, that’s not what your inbox looks like, but it’s  a pretty picture.  Your inbox is actually a file whose format depends on the MTA and most MTAs have their own format.

There’s many different MTAs available. Your system uses whatever the system administrator set up, so you don’t have to worry about it. A few of them are:

• sendmail (one of the originals!)
• exim (not as old as sendmail)
• qmail (older than exim but not as old as sendmail)

You don’t have to worry about the set up and management of these systems, someone else does that. If you want to learn, there’s books (and books and books) on the subject.

In the previous post, we talked about MX records. When we looked at gmail.com’s MX records, there were numbers before the hostnames. Those numbers are preferences, also known as MX preference.

How’s that for a technically boring term? MX preference means something when the computer is determining which of the records to use as a destination for my email. Let’s look at those results for gmail.com again:

gmail.com mail is handled by 40 alt4.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 20 alt2.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 30 alt3.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 10 alt1.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 5 gmail-smtp-in.l.google.com.

The one with the lowest number has the highest preference. How’s that for a contradictory sentence? But it’s how it works. This means that if I’m sending email to gmail.com, then the first place it’s going to try is the system gmail-smtp-in.l.google.com. If this fails, then it goes to the next in the list by preference, that is, alt2.gmail-smtp-in.l.google.com.

The geniuses (Not Dr. Strange and Ironman!) who designed this knew that people really wanted to get their email. Well, not spam email, but they didn’t consider that. They knew that people really wanted email and it should work, so they made it so what should happen is that the system sending the email should try each in order of MX preference.

Not everyone does it, of course. The Internet is a cooperative and it depends on people doing the ‘right thing’ or an approximation close enough to keep things running. Sometimes you’ll send an email and see a bounce from the mailer-daemon (not a real demon), but if you try again in just a few minutes, it’ll go through.

Either the other domain didn’t have a server up to receive the email or the system didn’t check through the MX preference list like it should have, or there was a real demon in the system. Any one of these could have caused the problem.  Especially the demon, demons are trouble.

Fred the Demon

I call mine Fred.

Disclaimer: There are no real demons in the Internet. …I think.

I get lots of email, some of it I want, some of it I’d rather not see. I’m sure that’s true for everyone with an email address. If I get the email, how does it get there? If I send email to my brother, how does my computer know how to get the email to him? It’s a bit of a complicated process, so we’re going to break it down and start by talking about Email and DNS.

One day, I decide I want to send an email to myfriend@example.com.

Why did I say example.com rather than a domain like gmail.com or hotmail.com? That’s because example.com is a domain that was set aside at the beginning of Internet time so people can use it in examples without referring to existing domains. It’s an example, just like its name.  It’s useful to use it because existing domains have been known to go away or change behavior, but example.com is always that, an example.

Ok, back to the subject at hand!

I’m sending email to myfriend@example.com and my first question is how does my email know where to send it?  My second question is, do I send it to example.com or somewhere else?

The geniuses that set up the Internet originally knew that the owner of a domain wasn’t necessarily the same person that ran the domain. The  domain computerlamp.net is what has this blog and while I own that domain, I don’t run things around here. I let DreamHost (Hi guys!) run this for me. That means that they handle my email, I don’t. All I do is read it.

DNS lets us know this by using the MX record. (MX means ‘mail exchanger’. They skipped the E when they made the acronym.)

The MX record for computerlamp.net is:

computerlamp.net mail is handled by 0 mx2.sub4.homie.mail.dreamhost.com.
computerlamp.net mail is handled by 0 mx1.sub4.homie.mail.dreamhost.com.

I did this using the command host.  Using the -t flag, my command was:

Alpha:~ computerlamp$ host -t MX computerlamp.net

If I wanted to know who handles the email for gmail.com, I’d do:

Alpha:~ computerlamp$ host -t MX gmail.com
gmail.com mail is handled by 40 alt4.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 20 alt2.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 30 alt3.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 10 alt1.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 5 gmail-smtp-in.l.google.com.

I got 5 responses back… with numbers in them! Next time, we’ll talk about what those numbers mean.

Ever noticed how a clock may not have the exact right time? Like it’s close, but not there? The same thing can happen with a computer. It relies upon its internal clock, but that can be almost, but not quite, right. Luckily, there’s a protocol to fix that (the unofficial motto of the Internet) called Network Time Protocol.

This lets you automatically make sure the clock on your computer is set to the right time. This is really important if you have many computers working together. Kind of like Tony Stark’s drones in the Age of Ultron movie. Suppose he sent them after Ultron but the clocks are all just a little bit off. Instead of blasting Ultron all at once, you’d get one shot, a few minutes later another, and then another, and.. see? that’s not a good thing. You want one BLAST to take Ultron (and HIS drones) out.

So, how does it work? To begin with, you have a really accurate clock, something like an Atomic Clock. That has an awesome name, right? Sounds like something used in a Doomsday weapon to make sure that no one can stop it, except maybe Super Man. It’s one of the most accurate clocks known, so that’s a good thing to ask.

There’s computers that synchronize directly to the clocks, so those are a good thing to ask too. Generally, there’s a list of servers on your system that your system will synchronize to. Network Time Protocol is designed to use the list and say ‘yo, other computer, tell me what time it is’ and then adjust your computer if it’s off.

The list of servers is kept in the file ntp.conf which is generally located at /etc/ntp.conf. Let’s look at it:

# /etc/ntp.conf, configuration for ntpd

driftfile /var/lib/ntp/ntp.drift
statsdir /var/log/ntpstats/

statistics loopstats peerstats clockstats
filegen loopstats file loopstats type day enable
filegen peerstats file peerstats type day enable
filegen clockstats file clockstats type day enable


# You do need to talk to an NTP server or two (or three).
#server ntp.your-provider.example

# pool.ntp.org maps to more than 300 low-stratum NTP servers.
# Your server will pick a different set every time it starts up.
#  *** Please consider joining the pool! ***
#  *** <http://www.pool.ntp.org/join.html> ***
server 0.debian.pool.ntp.org iburst
server 1.debian.pool.ntp.org iburst
server 2.debian.pool.ntp.org iburst
server 3.debian.pool.ntp.org iburst

There’s lots of information in there. The important part is the names after that word server. This computer uses servers by the name of 0.debian.pool.ntp.org 1.debian.pool.ntp.org, 2.debian.pool.ntp.org and 3.debian.pool.ntp.org to keep the time straight. This means that if one of those isn’t available, the Network Time Protocol will ask one of the other ones.
One of the interesting facts about the Network Time Protocol is that it’s one of the oldest protocols on the Internet. People have been trying to keep the times synchronized on their computers as long as they’ve been able to talk to other computers.