The Domain Name System (DNS) is a vast distributed database whose main job is to map hostnames to IP addresses. It also has an important role in email routing. In this chapter we’ll look at how MTAs in general use DNS and some of the DNS issues that relate to Postfix and its configuration. Keep in mind that there are two important aspects to your mail servers and DNS:
For sending mail, the system running your Postfix mail server must have access to a reliable DNS server to resolve hostnames and email-routing information.
For receiving mail, your domains must be configured correctly to route messages to your mail server.
Misconfiguration of DNS servers is a common source of problems in setting up email servers.