mailer.js

var nodemailer = require('nodemailer')

var transporter = nodemailer.createTransport("SMTP", {
  host: "smtp.domain.com",
  secureConnection: false,
  port: 587,
  auth: {
    user: "email@domain.com",
    pass: 'password'
  },
  tls: {
    ciphers:'SSLv3'
  }
})

const json = {
  'email@domain.com': 'John Doe'
}
const arr = Object.keys(json).map((key)=>{
  return {email: key, name: json[key]}
})

let total = arr.length
const send = ()=>{
  if (!arr[index]) {
    console.log('Finished')
    return
  }

  const {name, email} = arr[index]

  var mailOptions = {
    from: 'John Doe <email@domain.com>',
    to: `${name} <${email}>`,
    subject: 'This is my subject',
    text:
`
To ${name}

This is an email

John Doe.
`
  }

  transporter.sendMail(mailOptions, function(error, info){
    if(error){
      console.log(error)
    } else {
      console.log(`Message sent: ${name} <${email}> ${(index+1)} of ${total} (${(index/total * 100).toFixed(1)})%`)
    }
    index++
    setTimeout(()=>{
      send()
    },3000)
  })

}

send()