Frequently, systems wish to hide their true identity, serve as mail gateways, or receive and process mail addressed to `old' names by which they used to be known.
PSEUDONYMS specifies the list of all hostnames for which the local system will accept mail.
DEFAULT_HOST specifies the hostname that will appear in messages originating on the local host. It is important that this parameter be set to a valid value or all return mail will be undeliverable.