Custom routing plugin

customrouter.js

// store as plugins/customrouter.js

'use strict';

module.exports.title = 'Custom router';
module.exports.init = function(app, done) {
    // Set up custom MX for a delivery
    app.addHook('sender:fetch', (delivery, next) => {
        delivery.mx = [{
            priority: 0,
            exchange: app.config.hostname,
            A: [].concat(app.config.ip4 || []),
            AAAA: [].concat(app.config.ip6 || [])
        }];
        delivery.mxPort = app.config.mxPort;
        next();
    });

    done();
};

customrouter.toml

# store as config/plugins/customrouter.toml

[customrouter]
enabled="sender"
# MX hostname
hostname="example.com"
# MX IPv4, either IP string or an array of IP addresses
ip4="127.0.0.1"
# MX IPv6, optional
#ip6="::1"

Dear Andris,
I'm a beginner.
I can't figure out how to configure external relay for all outgoing mail. Here is what I want: http://www.postfix.org/postconf.5.html#relayhost
Like postfix relayhost, I want to route all my outgoing mails through another relay, say through an Cloud Antispam gateway.
On postfix, if I add just this line: relayhost = relay.symantec.com:25 all done.
How can I achieve this with ZoneMTA please? I have moved customrouter.js to plugins folder & customrouter.toml to config/plugins folder. Then added the lines below to production.json file inside plugins:
"customrouter": { "enabled": ["sender"], "host": "relay.symantec.com", "port": 25
Doesn't work as I can't figure it out. Please help me.