[Solved]-React onSubmit function doesn't hit Express endpoint (Nodemailer)-Reactjs

You are sending the form to /feedback as a post request but in the server side you you are having a post method with route /.So,The route /feedback is not in the express and so it throwed you 404 not found.Try changing the server route to /feedback.

​Did you make sure to include the middleware to protect web servers?

--Try this--

//middleware meant to protect web servers [CORS requests]
//can change '*' to 'http://localhost:3000' for local host testing

app.use((request, response, next) => {
 response.header("Access-Control-Allow-Origin", "*");
 response.header("Access-Control-Allow-Headers", "Content-Type");
 next();
 app.use(bodyParser.json())
 app.use(bodyParser.urlencoded){
 extended: false
}));