Не отправляется почта с формы

<?php
if($_REQUEST['first_name'] == '' || $_REQUEST['contact_email'] == '' ||  $_REQUEST['message'] == ''):
  return "error";
endif;
if (filter_var($_REQUEST['contact_email'], FILTER_VALIDATE_EMAIL)):
  $subject = 'Email со страницы teslaomsk.ru'; // Subject of your email
 
  // Receiver email address
  $to = 'info@teslaomsk.ru';  //Change the email address by yours
 
 
  // prepare header
  $header = 'From: '. $_REQUEST['first_name'] . " " .$_REQUEST['last_name'] . ' <'. $_REQUEST['contact_email'] .'>'. "\r\n";
  $header .= 'Reply-To:  '. $_REQUEST['first_name'] . " " .$_REQUEST['last_name'] . ' <'. $_REQUEST['contact_email'] .'>'. "\r\n";
  // $header .= 'Cc:  ' . 'example@domain.com' . "\r\n";
  // $header .= 'Bcc:  ' . 'example@domain.com' . "\r\n";
  $header .= 'X-Mailer: PHP/' . phpversion();
 
 
  $message .= 'Name: ' . $_REQUEST['first_name'] . " " .$_REQUEST['last_name'] . "\n";
  $message .= 'Email: ' . $_REQUEST['contact_email'] . "\n";
  $message .= 'Subject: ' . $_REQUEST['contact_subject'] . "\n";
  $message .= 'Message: '. $_REQUEST['message'];
 
  // Send contact information
  $mail = mail( $to, $subject , $message, $header );
 
  echo 'sent';
  else:
    return "error";
  endif; 
 
?>