Why should you care about address classes? This is how Postfix
decides what mail to accept, and how to deliver it. In other words,
address classes are very important for the operation of Postfix.
An address class is defined by three items.
The list of domains that are a member of the class: for
local domains, or all
The default delivery method. For example, the local or
smtp delivery agent. This helps to keep Postfix configurations
The list of valid recipient addresses for that address
class. The Postfix SMTP server rejects invalid recipients with
"User unknown in <name of address class here> table". This
helps to keep the Postfix queue free of undeliverable MAILER-DAEMON