Postfix: The Definitive Guide

Backup MX

In DNS, MX records refer to mail exchangers (see Chapter 6). MX records contain both host and priority (or preference) information for sending mail to a domain. A backup MX server is one that receives mail for a particular domain, but is not the preferred server to receive the mail. If the preferred server or servers are down, the backup MX server receives the mail and queues it until one of the more preferred servers comes back online. Figure 9-1 illustrates delivery to a backup host when the primary host is not available. The backup queues messages until the primary is back online, whereupon the backup can deliver messages to it.

Figure 9-1. Delivery to backup MX host

When your system is configured in DNS as a backup MX host, you don’t have to configure any special transport from your system to the primary system. Postfix uses the DNS records to determine how to route mail to the primary MX host. The only configuration required in Postfix is to indicate that it should receive mail for the domain by adding the domain name to the relay_domains parameter. When a sending MTA discovers that the primary mail system for a domain is down, it tries the next preferred one until it finds one that accepts delivery. If your system is a backup MX host, and the destination domain is listed in your relay_domains parameter, Postfix accepts the mail and queues it. Postfix periodically ...