Some emails not sent codeigniter - php

Question

I am trying to send mass emails via Codeigniter email library. However, only the first few emails are sent successfully.
Here is my Mail_model
<?php
defined('BASEPATH') OR exit('No direct script access allowed');
class Mail_model extends CI_Model{
public function __construct(){
parent::__construct();
$this->load->library('email');
}
public function send_mail($email, $firstname, $data){
$subject = $data['subject'];
$message = 'Dear '.$firstname.',<br /><br />'.$data['message'].'<br />'.DOMAIN_NAME;
//configure email settings
$config['protocol'] = 'smtp';
$config['smtp_timeout'] = 5;
$config['smtp_host'] = SMTP_HOST; //smtp host name
$config['smtp_port'] = SMTP_PORT; //smtp port number
$config['smtp_user'] = FROM_EMAIL;
$config['smtp_pass'] = FROM_PASSWORD; //$from_email password
$config['mailtype'] = 'html';
$config['charset'] = 'utf-8';
$config['wordwrap'] = TRUE;
$config['newline'] = "\r\n"; //use double quotes
$config['validation'] = TRUE;
$this->email->initialize($config);
//send mail
$this->email->from(FROM_EMAIL, DOMAIN_NAME);
$this->email->to($email);
$this->email->subject($subject);
$this->email->message($message);
//echo json_encode($this->email->print_debugger());
$this->email->send();
}//end method send_mail
}//end class
Here is the bit of code from my controller that I am calling send_mail function from the model
$customers = $this->customer_model->get_customers();
foreach($customers as $customer){
$this->mail_model->send_mail($customer['email'], $customer['firstname'], $this->input->post());
sleep(10);
}
I added sleep(10) at the end of the loop to see if everything will work fine but it does not help. I am using Gmail.