`relayhost`

Typically, you use `Deliver directly` so that Postfix will forward any outgoing mail not destined for users of your system to the given host.

If you want to forward all mail to an intranet mail gateway (for example if your intranet has a firewall), except for mail for the local machine, you will setup the value to your mail gateway. Most of the time, organization has set up internal MX records, so using directly your domain name will bind to the mail gateway machine.