I am running a server using Nodemailer. Sometimes the api service respond me with 421.

"{"errorCode":421,
"erroText":"Error occurs",
"error":{"code":"EENVELOPE","response":"550 5.1.0 <xxx@xxxxxx.it> Connessione da 100.27.27.90 temporaneamente rifiutata / Connection from 100.27.27.90 temporarily rejected","responseCode":550,"command":"MAIL FROM"}}"

This is my code in node

var transporter = nodemailer.createTransport({
  host: "smtps.aruba.it",
  logger: true,
  debug: true,
  secure: true,
  port: 465,
  auth: {
    user: "xxx@xxxxx.it",
    pass: "xxxxx",
  },
  tls: {
    minVersion: "TLSv1",
    ciphers: "HIGH:MEDIUM:!aNULL:!eNULL:@STRENGTH:!DH:!kEDH",
  },
});

let mailOptions = {
  from: '"xxx" <xxx@xxx.it>',
  to: email,
  subject: "Riepilogo prenotazione",
  html: `...`,
};

transporter.sendMail(mailOptions, function (err, data) {
  if (err) {
    res.json({
      errorCode: 421,
      erroText: "Error occurs",
      error: err,
    });
  } else {
    res.json({
      errorCode: 200,
      status: "Appointment Created!",
    });
  }
});

transporter.close();

If I make 2 or 3 calls in a row, the service gives me error 421 with this description