Use the Mail Options Web page to specify how to manage emails and to define configuration information used in processing incoming and outgoing emails.
If you specify a mailbox gateway, the system sends all outbound emails to the gateway for delivery to the destination.
If you do not specify a mailbox gateway, the system uses the configured Domain Name server to look up the host.domain portion of the outgoing user@host.domain email address.
If the DNS lookup fails, the system checks the administered host files entries for host.domain.
If all the methods fail, the message is marked as undeliverable and returned to the sender.