although the Vercel official documents say it is possible, but the officail docs also say it is not recommended. plz take a look at this.
https://vercel.com/guides/using-express-with-vercel#re-thinking-our-application

Vercel magic is designed for serverless environments like AWS Lambda and Cloudflare Worker. but express cannot leverage the Vercel magic though.

Is there any way to convert my express code into serverless function?

if it a handfull codes, you can migrate them without using Express, otherwise you want to stick with self-hosted env.

Here's my code:

const express = require("express") 
 const bodyParser = require("body-parser") 
 const nodemailer = require("nodemailer") 
 const cors = require("cors") 
  
 require("dotenv").config() 
  
 const app = express().use(bodyParser.json()) 
  
 const corsOptions = { 
     origin: '*', 
     credentials: true, 
     optionSuccessStatus: 200, 
 } 
  
 app.use(cors(corsOptions)) 
  
 const myEmail = process.env.EMAIL 
 const password = process.env.PASSWORD 
  
 app.post("/", (req, res) => { 
  
     try { 
         const { name, email, message } = req.body 
  
         const transporter = nodemailer.createTransport({ 
             service: "gmail", 
             auth: { 
                 user: myEmail, 
                 pass: password 
             }, 
             host: "smtp.gmail.com", 
             port: 465, 
             secure: true, 
         }) 
  
         const mailOptions = { 
             from: myEmail, 
             to: myEmail, 
             subject: "Portfolio Email", 
             text: `Name: ${name}\n\nEmail: ${email}\n\nMessage: ${message}`, 
         } 
  
         transporter.sendMail(mailOptions, (error, info) => { 
             if (error) { 
                 console.error(error) 
                 res.status(500).json({ error: "Failed to send email" }) 
             } else { 
                 console.log("Email sent:", info.response) 
                 res.status(200).json({ message: "Email sent successfully" }) 
             } 
         }) 
     } catch (error) { 
         console.log(error) 
     } 
 }).listen(process.env.PORT || 4000, () => { 
     console.log(`Server is running on port ${process.env.PORT || 4000}`) 
 })

Is it possible?

technically it is possible but some caveat. AWS lambada has wall clock time cap 10 seconds, so if the sendMail server takes 11 seconds it fails.

if there is asycn version of sendMail, you can deploy it to Edge Runtime, where it is measured by CPU time 50ms instead of wall clock time. meaning it runs longer than 10 sec. but you can not use 'require', use 'import'

so all i need to do is add "type": "module" and add async to my function?

im not sure. you want to try it to see how it works as it is really small piece of code.
just keep in mind that Vercel has two runtimes: Serverless(AWS Lambada) and Edge(Cloudflare Worker)
the former is a Node.js, while the other is a subset of Node.js named V8 Isolates.
so some of libraries like axios does not work in Edge.

Okay. Thanks!

Last question, I should remove the .listen in production right?

yes

even in dev. don't run a server from Next.js framework. its a framework. Vercel takes care of it.

So what endpoint should I post here when using serverless function?

        axios.post("https://stormy-onesies.cyclic.cloud", { 
             name: values.name, 
             email: values.email, 
             message: values.message 
         })

go through the official docs about api
https://nextjs.org/docs/app/building-your-application/routing/route-handlers

@tafutada777 hello, I've made serverless function to work in my project but only in dev. When I deploy it in prod and tried sending email, it says email sent successfully but I didn't receive it in my gmail. However, when I'm running localhost with my app and tried sending email, i can receive the message perfectly fine in my gmail. I have set up my local env variables already in vercel and called it using process.env . Also, when I check the console when sending in prod, it says email sent successfully but again I didn't receive the message in my gmail. Do I still need to do additional steps to deploy serverless function in prod?

broo thank you so much! all I need is just add await for the sendMail method