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.
