In this article I will explain the basics of SMTP routing using DNS and smart hosts. I decided to write this article after spending some time with a number of administrators explaining not only how SMTP mail works, but how the mail actually “gets there” and why.

Let’s assume I have two internet mail domains, noddy.com and outlookexchange.com, and lets further assume my friend Steve at outlookexchange wants to email me. So he writes his email and hit’s SEND, and Exchange takes over, routes the mail and delivers it – right? Yes, but how? Let’s look at an example of DNS routing –we’ll cover “smart hosts” a bit later.

First - Exchange examines the email header – makes a routing decision that this mail should go out to the internet and hands it off to the internet facing host. This host will again examine the email header and see that the intended recipient is @noddy.com. Some routing information is required here, since the outlookexchange.com mail host doesn’t necessarily know how to send mail to the mail servers at noddy.com, or even what their names and IP addresses are.

This information is stored in DNS as Mail Exchanger records – MX records for short. An MX record will normally contain an “A” record – i.e. a name of a mail host which will resolve to an IP address.  E.g. mail.noddy.com

The sending host issues a DNS query for noddy.com’s MX records and subsequently has an idea of where to send mail to, establishes a connection to port 25 of the host listed as the mail exchanger for the domain, and sends the mail across. For a better idea of how THAT happens, have a look here.

Lets look at the mail records for outlookexchange.com to get a better idea of this:

Run nslookup from the command prompt.

Type outlookexchange.com and hit ENTER.

Filter through the results until you get to the MX record:

outlookexchange.com     MX preference = 10, mail exchanger = postoffice.omnis.com

Do this for your own domain and see what you get.