Code PHP ne pas envoyer de mail client

<?php
require_once "/usr/lib/php/pear/Mail.php";

$from = " <>";
$to = "test1 <>";
$subject = "Hi!";
$body = "Hi,\n\nHow are you?";

$host = "";
$username = "";
$password = "test_123";

$headers = array ('From' => $from,
 'To' => $to,
 'Subject' => $subject);
$smtp = Mail::factory('smtp',
 array ('host' => $host,
  'auth' => true,
  'username' => $username,
  'password' => $password));

$mail = $smtp->send($to, $headers, $body);

if (PEAR::isError($mail)) {
 echo("<p>" . $mail->getMessage() . "</p>");
} else {
 echo("<p>Message successfully sent!</p>");
}
?>