IIS Aid

Configuring your DNS records

Before we install your mail server it is a good idea to ensure you DNS records are correctly setup for email. DNS is vital to the operation of the Internet, and in a nutshell what it does is create a link between a human friendly domain name (e.g. example.com) and an IP addresses. When a user enters a web address into their browser, that domain is then resolved to an IP address which is then used to communicate. When hosting your own email server there are three very important types of DNS records to consider, and these are A, PTR and MX records.

An A record, or Address record is one of the most common types of DNS records you'll come across. Its purpose is to simply create a link between an IP address and a domain name. For example for the domain name of example.com you would have an A record that contained the IP address of the server that was hosting example.com. With A records the domain must be unique, but the IP address does not. This means you can have as many domain names pointing to the same IP address as you want, and is used extensively these days for both Email and Web hosting. In regards to your Email environment, it is best to create a separate A record that identifies your email server. This can be something like mail.example.com or similar. You'll need to create an A record for each server you plan on using as a mail server.

A PTR, or Pointer Record is unique in DNS in that you can only have one PTR address per IP address. This is because the function of a PTR record is to resolve a human friendly name from an IP address, instead of the other way around. Because you are resolving a domain name from an IP address, there can only be one authoritative record. If possible it is best to set your PTR record of the IP your email will be hosted from to be the same as your A record (e.g. mail.example.com), and you may need to contact your hosting company to do this for you.

Finally we have the MX, or Mail eXchange record which is used by other mail servers to direct email to the right place. Unlike an A or PTR record an MX record is not associated with an IP address in anyway, but instead contains the human friendly name of the A record you wish to use for your mail server. This might sound a little redundant as all it is doing is pointing from one record to another, but MX records also have one other important function. This is to establish the pecking order of your email servers by using a preference field in which you can enter a numerical value (the lower the number, the more important that server is) to define in what order other email servers should contact your email servers. If the email server with the highest preference (lowest number in preference field) is not contactable, then incoming email servers will simply use the server identified by the MX record with the next highest preference.