Sending and receiving email is one of the things we spend most of our time at work on. Therefore email has become one of the most important pieces in corporate infrastructure - we can't do without.

This means that maintenance, capacity and availability are of the highest priority. Recently I have been involved in a project adding capacity to a running mail environment. By the time the project was setup, the current mailservers were piling up to 80,000 mails in their queue's. Per system, mind you ! Therefore, the solution was simple, we simply needed to add capacity. Two additional mailservers were setup and configured. In order to start using the new systems, they needed to be configured in DNS (nameserver) in order to become part of the mailinfra and receive load.

In itself, an easy task. But it made me realize the power of MX records within DNS : You can influence the percentage of mail load going to a specific mailserver by setting MX priority. That way, you can ensure a smooth integration, as you can have the added servers handle only a little bit of load at first to see if that works okay. You can also infuence handled domains for a specific server by adding it, or not adding it to the domain's DNS zone.

The relative priority of an MX server is determined by the preference number present in the DNS MX record. When a remote client (typically another mail server) does an MX lookup for the domain name, it gets a list of servers and their preference numbers.(Answers.com)

A good and clear explanation is here.