Nodemailer gmail connection closed with a lot of emails

I have exactly the same problem as this post: Send multi-user emails with nodemailer and gmail

When sending too many emails with nodemailer and gmail, I get a 421 error referring to too many concurrent sessions.

What can I do to avoid opening too many sessions?

I already go to google to confirm that I was not blocked by any restrictions (I did not get to the daily mail number and there is no mail / minute limit). I tried to wait for each mail to be sent before sending a new one; create and close a new transport on every mail, but I keep getting this error after about 100th mail.

Here's the complete error:

{ [Error: Mail command failed: 421 4.7.0 Try again later, closing 
connection. (MAIL) e17sm2124566ede.14 - gsmtp]
code: 'EENVELOPE',
response: '421 4.7.0 Try again later, closing connection. (MAIL) 
e17sm2124566ede.14 - gsmtp',
responseCode: 421,
command: 'MAIL FROM' }

      

        
        
        
      

    

And my code:

Nodemailer settings:

function setMailTransport () {
  return nodemailer.createTransport(smtpTransport({
    service: 'gmail',
    ignoreTLS: true,
    auth: {
      xoauth2: xoauth2.createXOAuth2Generator({
        user: 'xxxxxx',
        clientId: 'xxxxxx',
        clientSecret: 'xxxxxx',
        refreshToken: 'xxxxxx'
      })
    }
  }))
}

      

        
        
        
      

    

Sending unique mail:

async function sendEmail (mail) { 
  // mail is an object {from, to, subject, text, html}
  const transport = setMailTransport()
  try {
    await transport.sendMail(mail)
    await transport.close()
    return 1
  } catch (err) {
    console.log(err)
    await transport.close()
    return 0
  }
}

      

        
        
        
      

    

Recursive async / await function to wait for mail to be sent before sending a new message:

async function sendAlerts (mails, index, numberOfMailSent) {
  // mails is an array of mail object, index start at 0  
  // numberOfMailSent is just a counter to know how many mails have been sent
  if (index >= mails.length) return numberOfMailSent
  const mail = mails[position]
  const newMailSent = await sendEmail(mail)
  return sendAlerts(mails, index + 1, numberOfMailSent + newMailSent)
}

      

        
        
        
      

    

Any idea where I could be wrong or in some other way send over 100 emails?