Unable to send more than 10 emails with nodemailer with yahoo smtp server

Question

I have been trying to send bulk emails (around 10 - 20 emails a day) with nodemailer. But I found that my code always generate 554 6.6.0 Error sending message for delivery. May I get some insights please? For what is worth, my transporter setting is as follows:

const transporter = {
  host: "smtp.mail.yahoo.com",
  port: 587,
  service: "yahoo",
  secure: false,
  auth: {
    user: [email protected],
    // create in the yahoo account - security settings
    pass: user.password,
  },
  logger:true,
  debugger:true
};