Canonical versus hosted versus other domains

Most Postfix systems are final destination for only a few domain names. These include the hostnames and [the IP addresses] of the machine that Postfix runs on, and sometimes also include the parent domain of the hostname. The remainder of this document will refer to these domains as the canonical domains. They are usually implemented with the Postfix local domain address class, as defined in the ADDRESS_CLASS_README file.

Besides the canonical domains, Postfix can be configured to be final destination for any number of additional domains. These domains are called hosted, because they are not directly associated with the name of the machine itself. Hosted domains are usually implemented with the virtual alias domain address class and/or with the virtual mailbox domain address class, as defined in the ADDRESS_CLASS_README file.

But wait! There is more. Postfix can be configured as a backup MX host for other domains. In this case Postfix is not the final destination for those domains. It merely queues the mail when the primary MX host is down, and forwards the mail when the primary MX host becomes available. This function is implemented with the relay domain address class, as defined in the ADDRESS_CLASS_README file.

Finally, Postfix can be configured as a transit host for sending mail across the internet. Obviously, Postfix is not final destination for such mail. This function is available only for authorized clients and/or users, and is implemented by the default domain address class, as defined in the ADDRESS_CLASS_README file.