Reactjs – Email not delivering in production but delivering in development

I have exactly the same error in deployed express.js backend on Vercel :

Error: Invalid login: 535 5.7.3 Authentication unsuccessful [MN2PR06CA0003.namprd06.prod.outlook.com 2024-05-31T14:43:25.191Z 08DC8174A56E57CA]
at SMTPConnection._formatError (/var/task/node_modules/nodemailer/lib/smtp-connection/index.js:798:19)
at SMTPConnection._actionAUTHComplete (/var/task/node_modules/nodemailer/lib/smtp-connection/index.js:1577:34)
at SMTPConnection.<anonymous> (/var/task/node_modules/nodemailer/lib/smtp-connection/index.js:1531:18)
at SMTPConnection._processResponse (/var/task/node_modules/nodemailer/lib/smtp-connection/index.js:982:20)
at SMTPConnection._onData (/var/task/node_modules/nodemailer/lib/smtp-connection/index.js:763:14)
at SMTPConnection._onSocketData (/var/task/node_modules/nodemailer/lib/smtp-connection/index.js:195:44)
at TLSSocket.emit (node:events:519:28)
at addChunk (node:internal/streams/readable:559:12)
at readableAddChunkPushByteMode (node:internal/streams/readable:510:3)
at Readable.push (node:internal/streams/readable:390:5) {
code: 'EAUTH',
response: '535 5.7.3 Authentication unsuccessful [MN2PR06CA0003.namprd06.prod.outlook.com 2024-05-31T14:43:25.191Z 08DC8174A56E57CA]',
responseCode: 535,
command: 'AUTH LOGIN'
}

Based on the code below using promise to wrap nodemailer.sender as suggested in: Nodemailer in vercel not sending email in production

router.post("/", async (req, res) => {

  const { firstname, lastname, email, phone, message, ownCopy } = req.body;

  const contactFields = {
    firstname,
    lastname,
    email,
    message,
    ownCopy,
  };

  if (phone) {
    contactFields.phone = phone;
  }

  const emailContent = [
    `Nom: ${firstname} ${lastname}`,
    `Email: ${email}`,
    phone ? `Téléphone: ${phone}` : "",
    `Message: ${message}`,
  ]
    .filter(Boolean)
    .join("nn");

  let transporter = nodemailer.createTransport({
    host: "smtp-mail.outlook.com",
    port: 587,
    secure: false,
    auth: {
      user: process.env.OUTLOOK_EMAIL,
      pass: process.env.OUTLOOK_PASSWORD,
    },
  });

  await new Promise((resolve, reject) => {
    // verify connection configuration
    transporter.verify(function (error, success) {
      if (error) {
        console.log(error);
        reject(error);
      } else {
        console.log("Server is ready to take our messages");
        resolve(success);
      }
    });
  });

  const newContact = new Contact(contactFields);
  await newContact.save();

  let mailOptions = {
    from: process.env.OUTLOOK_EMAIL,
    to: process.env.OUTLOOK_EMAIL,
    cc: ownCopy ? email : "",
    subject: "New Message from Contact Form",
    text: emailContent,
    //  html: "<b>Hello world?</b>",
  };

  await new Promise((resolve, reject) => {
    transporter.sendMail(mailOptions, (error, info) => {
      if (error) {
        reject(error);
        res.status(500).json({
          result: false,
          error: error,
        });
      }
      resolve(info);
      console.log("Message sent: %s", info.messageId);
      console.log("Preview URL: %s", nodemailer.getTestMessageUrl(info));
      res.status(200).json({ result: true, newContact });
    });
  });

Any advice on this problem?