SMTP Error: 454 4.7.0 Too many login attempts, please try again later

I had the same issue. When I checked the Mail queue there were many unprocessed mails in the queue. So I deleted the bulk mails and restarted the instance. Once the Mail Queue is cleared then it started to send mails as usual.

Hope this will be useful for anybody to have the above issue.

It is because you are attempting to create a new smtp connection for each email. You need to use SMTP pool.

Please see:

DELIVERING BULK MAIL

POOLED SMTP

Pooled smtp is mostly useful when you have a large number of messages that you want to send in batches or your provider allows you to only use a small amount of parallel connections.

If you are using Node-mailer:

const transporter = nodemailer.createTransport({
    host: 'smtp.gmail.com',
    port: 465,
    secure: true,
    pool: true, // This is the field you need to add
    auth: {
       user: '[email protected]',
       pass: 'your_password' 
}});

Then, you need to close the pool once you send all the emails.

transporter.close();