const nodemailer = require("nodemailer");
  const smtpTransport = nodemailer.createTransport({
    service: "Gmail",
    auth: {
      user: "...@gmail.com",
      pass: "..."
    }
  });

  router.post('/...', (req, res) => {
  ...
    rand = Math.floor((Math.random() * 100) + 54);
    host = req.get('host');
    link = "http://" + host + "/api/users/verify?id=" + rand;
    let mailOptions = {
      from: smtpTransport.user,
      to: req.body.user_email,
      subject: "Please confirm your Email account",
      html: "Hello,<br> Please Click on the link to verify your email.<br><a href=" + link + ">Click here to verify</a>"
    }
    smtpTransport.sendMail(mailOptions, (err, res) => {
      if (!err) {
        res.status(200).json({
          message: 'Email sent successfully',
        });
      } else {
        return res.status(500).json({
          error: err
        });
      }
    });
    ...
  });

router.get('/...', (req, res) => {
    if ((req.protocol + "://" + req.get('host')) == ("http://" + host)) {
        console.log("Domain is matched. Information is from Authentic email");
        if (req.query.id == rand) {
            console.log("email is verified");
            res.end("<h1>Email " + mailOptions.to + " is been Successfully verified");
        } else {
            console.log("email is not verified");
            res.end("<h1>Bad Request</h1>");
        }
    } else {
        res.end("<h1>Request is from unknown source");
    }
});