Form works fine on localhost but throws 404 once deployed - node.js

Question

The solution seems to be in front of eyes, but everything I tried so far didn't work. I'm using nodemailer to send e-mail through a form and it works fine on local host but when I deploy it, it doesn't work. I think it has something to do with the routing but I can't figure it out. What should I have on "action" and "app.post"? I'm using a subdomain on inmotion to test the form.
const nodemailer = require ('nodemailer')
const express = require ('express')
const bodyParser = require ('body-parser')
const app = express()
app.use (express.static('./public'))
app.use (bodyParser.json())
app.use (bodyParser.urlencoded ({extended: true}))
const transporter = nodemailer.createTransport({
host: "biz260.inmotionhosting.com",
port: "465",
secure: true,
auth: {
user: "test#learningtodom.com",
pass: "xxxxxx"
}
})
function sendEmail(mail) {
var mailOptions = {
from: "test#learningtodom.com",
to: "pablo.football.coach#gmail.com",
subject: "Mail",
html: mail.body
}
transporter.sendMail (mailOptions, function (err, info){
if (err) {
console.log (err)
}
else {
console.log('Done' + info.response)
}
})
}
app.post ('/send_email_form', (req, res) => {
mail = {
to : req.body.name,
subject: req.body.second,
body: req.body.second
}
sendEmail(mail)
res.redirect ('/')
})
app.listen (8100, () => {
console.log ('Server Running at 8100')
})
Form
<form action="/send_email_form" method="POST">
<div class="form-row">
<div class="col-md-4 mb-3">
<label for="validationServer01">First name</label>
<input type="text" name ="name" class="form-control is-valid" id="validationServer01" placeholder="First name" value="Mark" required>
<div class="valid-feedback">
Looks good!
</div>
</div>

Answer 1

It may be your port settings, 8100 is not a normal port to be open on a production server. It is most common to see regular HTTP traffic on port 80, and HTTPS traffic on port 443.
If it is possible to open port 8100 on your remote server, you should do that to check if your server is reachable on that port. It can help to add a simple status checking endpoint such as
app.get ('/status', (req, res) => {
res.json({ status: 'ok' })
})
and then confirm that you can access that by trying http://[yourdomain]:8100/status in a browser.
I highly recommend reading your port number from your environment variables (like node.env.PORT) so that you can easily serve locally on 8100 but then switch to 80 and/or 443 when deployed on a remote server.
Once you confirm that you can hit a GET/status route, then any other issue you have will be to do with your nodemailer setup.
Also, be aware that using a library like nodemailer may make your mail likely to go into your spam folder. Paid services like Sendgrid and Mailgun work hard to ensure the authenticity of email senders, so emails sent using them are much more likely to be delivered as not spam.
If you are going to be the only recipient of this mail, and can find it in your spam folder, make sure to mark it as not spam, and try to let your inbox know that you trust emails from this address.
Good luck, hope this helps :)