Relay hosts configuration (or mail routes) formerly known as Smarthost in ns7 is a planned feature, still not implemented. In future releases it will be configured from UI.
Meanwhile thank you for the Postfix customization example. You read the docs, pure joy for me.
Mark, I’m really happy to read again from you (There will be a talk about ARM and NS8 at our community meeting Saturday afternoon at Fosdem, but I didn’t say it).