The "Hostname" option specifies the "internet hostname" of this mail system. The default is to use the fully-qualified domain name from gethostname(). The "Hostname" option is used as a default value for many other configuration parameters. This option sets the "myhostname" postfix variable.
The "Domain" option specifies the local internet domain name. The default is to use the contents of "Hostname" minus the first component. The "Domain" option is used as a default value for many other configuration parameters. This option sets the "mydomain" postfix variable.