Up until now, we've mostly considered Postfix in its role as the end node for email messages. That is, messages that arrive at the Postfix system are, for the most part, delivered to the local system. But it's also common to find Postfix serving as an intermediate node on the path a message follows to its ultimate destination. In this chapter we'll look at some of the configuration options for Postfix as a client in MTA-to-MTA communications.
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 ...