server.js

var express = require('express');
var nodemailer = require('nodemailer');
var app = express();
const { transporter } = require('./transporter');
/*
    Here we are configuring our SMTP Server details.
    STMP is mail server which is responsible for sending and recieving email.
*/
var smtpTransport = nodemailer.createTransport({
  service: 'gmail',
  host: 'smtp.gmail.com',
  auth: {
    user: '',
    pass: '',
  },
});

/* const transporter = nodemailer.createTransport({
  service: 'gmail',
  auth: {
    type: 'OAuth2',
    user: process.env.USER,
    clientId: process.env.CLIENT_ID,
    clientSecret: process.env.CLIENT_SECRET,
    refreshToken: process.env.REFRESH_TOKEN,
    accessToken: process.env.ACCESS_TOKEN,
  },
}); */
/*------------------SMTP Over-----------------------------*/

/*------------------Routing Started ------------------------*/

app.get('/', function (req, res) {
  res.sendFile('index.html', {
    root: __dirname,
  });
});
app.get('/send', function (req, res) {
  var mailOptions = {
    from: 'Kathirr007 <kathirr007@gmail.com>',
    to: req.query.to,
    subject: req.query.subject,
    text: req.query.text,
    html: req.query.htmlContent,
  };
  console.log(mailOptions);
  transporter.sendMail(mailOptions, function (error, response) {
    if (error) {
      console.log(error);
      res.end('error');
    } else {
      console.log('Message sent...');
      res.end('sent');
    }
  });
});

/*--------------------Routing Over----------------------------*/

app.listen(3000, function () {
  console.log('Express Started on Port 3000');
});