contact_backend.js

let app = require('express')();
app.use(require('body-parser').urlencoded());

const CONTACT_ADDRESS = '[email protected]';

var mailer = require('nodemailer').createTransport({
  service: 'Gmail',
  auth: {
    user: process.env.GMAIL_ADDRESS,
    pass: process.env.GMAIL_PASSWORD,
  }
});

app.post('/contact', function(req, res) {
  mailer.sendMail({
    from: req.body.from,
    to: [CONTACT_ADDRESS],
    subject: req.body.subject || '[No subject]',
    html: req.body.message || '[No message]',
  }, function(err, info) {
    if (err) return res.status(500).send(err);
    res.json({success: true});
  })
});

app.listen(3000);