gatsby form success page only allow access from form code example

Example: gatsby send email using php

CopyhandleForm.js: copy code to clipboardconst bodyParser = require("body-parser")const express = require("express")const nodemailer = require("nodemailer")const app = express()app.use(bodyParser.urlencoded())const contactAddress = "[email protected]"const mailer = nodemailer.createTransport({  service: "Gmail",  auth: {    user: process.env.production.GMAIL_ADDRESS,    pass: process.env.production.GMAIL_PASSWORD,  },})app.post("/contact", function (req, res) {  mailer.sendMail(    {      from: req.body.from,      to: [contactAddress],      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)