Mail

Overview

Some notes on how email works... or how to send emails using nslookup and telnet.

Sending

So you want to send nicolashubbard@hotmail.com an email. How is this done?

A mail client is used to compose the email - the memo header and body. I normally use use Outlook Express. It's configured to send SMTP to a local server "gateway" on port 25. 

SMTP

How does SMTP work? Here's the RFC.

Simple Mail Transfer Protocol routing

How does the Simple Mail Transfer Protocol (SMTP) server know where to send the email?

Machines which are physically connected to the Internet each have a unique IP address. DNS is a distributed  database of mappings between machine names and their IP addresses.

In order to communicate with a system, one machine must get the IP-address of the machine it wants to talk to from the DNS first. These are stored in the DNS in what are known as "A" records.

However, there are many named machines that are not always connected physically to the Internet. In order to get mail to these boxes, the DNS contains an "MX" record.

The "MX" record, instead a dotted IP-address, holds the name of the system which does have an IP-address and knows how to get mail to the real destination system. 

MX-records also have a number which is a "Precedence" number. They are tried in order from the lowest, but always skipping any MX-records which point to itself. 

NSLOOKUP

I can obtain the name of an SMTP server at hotmail.com by using nslookup:

I can now think about sending an email to nicolashubbard@hotmail.com, doing it manually.

Here I'm using telnet to connect to mc1.law13.hotmail.com on port 25. I've turned on local echo. My typing is highlighted in green.

The Result

Off I go to Hotmail and get the email:

in the body now!
.
not the end

It works! Hotmail doesn't let me see the memo header, and the sender email is <unknown>.

NSLOOKUP - MX records

Driving this app, like anything that has an ancestry in the UNIX world, is terse. That's being polite.

If your pc is connected to the internet, you can use nslookup to find MX records.

Run nslookup in a command shell - or whatever Bill calls Dos boxes these days.

Once nslookup is running, it prints the name of your pc's default name server and the IP address for that machine, then a > character as a prompt and awaits input:

To persuade nslookup to look up only MX records, use the set command:

Now look up some real hosts and domains. Here I look at hotmail.com:

Note the trailing dot that tells nslookup that the local, default domain should not be appended prior to the lookup. Isn't that so UNIX! 

The first two lines again show the name and IP address of the local DNS server. Skipping 2 lines, the next four show that the domain hotmail.com has four MX records. Mail addressed to that domain is sent to the machine with the lowest preference (cost). If that machine is down (or not accepting mail), the message is sent to the machine with the next higher cost. The last lines show the IP addresses (A records) for those machines.

Our domains, and the "any" command