When sending mail, Postfix uses system resolvers, which are programs or libraries that make requests for DNS information. To receive mail, the DNS for your domain must be configured to route messages to your Postfix server. This section looks at DNS issues both for sending and receiving mail.
The Postfix SMTP delivery agent must be able to obtain IP address and MX records for mail-routing information. Postfix must make at least two DNS lookups: one to get the MX hostname and one to get the IP address for that hostname. Since Postfix uses the normal operating system resolver libraries for its DNS queries, the system that runs Postfix must have access to a DNS server. The DNS server does not have to be on the same system, although for most circumstances it should be.
If your system does not seem to be resolving domain names correctly, there are three common command-line tools that you can use to troubleshoot the problem: nslookup , dig, and host. You should check your system documentation to see which of these tools is available on your server and how to use them. You can use these tools to query all types of resource records for a domain, including the MX record that Postfix needs in order to successfully deliver mail to a domain.
DNS problems might stem from your own system's configuration or a problem with the DNS server configuration for the domain Postfix is trying to send mail to. When you are troubleshooting a problem, it is very important ...