Relay Host

If the machine needs to pass mail to another mail server to get out to the Internet, the hostname of the mail server should be defined as the Relay Host. If a Relay Host is defined, mail to all domains not defined as a mail route or virtual domain will be forwarded to the machine defined as Relay Host. Generally, this option is used to relay outgoing mail.

In the above screen-shot example, the machine hostname is set as
mailbox.corp.guardiandigital.com and it forwards all outgoing mail to the machine
smtp.corp.guardiandigital.com. smtp.corp.guardiandigital.com sends the mail out to the Internet.

You can set the Relay Host and Backup Relay Host in the General Configuration section. The machine will forward mail to Backup Relay Host if the Relay Host is not available or doesn't accept mail. Note that Relay Hosts and Backup Relay Hosts should have corresponding DNS entries. You should be able to resolve them to the corresponding IP addresses.