index.js

const nodemailer = require("nodemailer");

exports.handler = function (event, context) { //eslint-disable-line
  const transport = nodemailer.createTransport({
    host: "smtp.mailtrap.io",
    port: 2525,
    auth: {
      user: "xxxxxxx", //reemplazar por tu usuario de Mailtrap
      pass: "xxxxxxx" //reemplazar por tu password de Mailtrap
    }
  });

  const { user, subject, message } = event.arguments;

  const mailOptions = {
    from: 'noreply@cursosdesarrolloweb.es',
    to: user.email,
    subject: subject,
    html: message
  };

  // send mail with defined transport object
  transport.sendMail(mailOptions, (error, info) => {
    if (error) {
      context.done(error);
    }
    context.done(null, event);
  });
};