📨 Building your Email API with Fastmail for free! (https://dev.to/creuserr/building-your-email-api-with-fastmail-for-free-h91)

index.js

const { createTransport } = require("nodemailer")
require("dotenv").config()

module.exports = (req, res) => {
  // if the client is within the server,
  // you can remove this header to
  // prevent outside access.
  res.setHeader("Access-Control-Allow-Origin", "*")
  res.setHeader("Content-Type", "text/plain")
  if(req.method != "POST") {
    return res.status(400).end("400 Bad Request: Invalid method")
  }
  if(req.body.length < 5) {
    return res.status(400).end("400 Bad Request: Body is too short (min 5)")
  }
  var reciever = req.headers["x-mail-reciever"]
  var subject = req.headers["x-mail-subject"]
  if(reciever == null || subject == null) {
    return res.status(400).end("400 Bad Request: Incomplete headers")
  }
  const sender = process.env.EMAIL
  const password = process.env.PASSWORD
  try {
    var transporter = createTransport({
      host: "smtp.fastmail.com",
      port: 465,
      auth: {
        user: sender,
        pass: password
      }
    })
    const options = {
      from: sender,
      to: reciever,
      subject,
      html: req.body
    }
  } catch(e) {
    res.status(500).end(`500 Internal Error: Failed to initialize transporter: ${e}`)
  }
  try {
    transporter.sendMail(options, function(e, i) {
      if(e) {
        res.status(400).end(`400 Bad Request: Failed to send mail: ${e}`)
      } else {
        res.status(200).end("200 Success")
      }
    })
  } catch(e) {
    res.status(400).end(`400 Bad Request: Failed to send mail: ${e}`)
  }
}

vercel.json

{
  "version": 2,
  "builds": [{
    "src": "api/index.js",
    "use": "@vercel/node"
  }],
  "rewrites": [{
    "source": "/api/(.*)",
    "destination": "/api/index.js"
  }],
  "headers": [{
    "source": "/api/index.js",
    "headers": [{
      "key": "Access-Control-Allow-Origin",
      "value": "*"
    }, {
      "key": "Access-Control-Allow-Methods",
      "value": "POST"
    }]
  }]
}