error mail(): SMTP server response: 550 5.7.1 Unable to relay for ...

<?php  
$subject = "Contact Form Website";
$to = "[email protected]";
$umail = $_POST['u_mail'];

$Headers = "MIME-Version: 1.0\r\n" ;
$Headers .= "Content-type: text/html; charset=UTF-8\r\n" ;
$Headers .= "From: ".$_POST['u_mail']." <".$_POST['u_mail'].">\r\n" ;
$Headers .= 'Cc: [email protected]' . "\r\n";
$Headers .= "Reply-to: ".$_POST['u_mail']." <".$_POST['u_mail'].">\r\n" ;
$Headers .= "X-Priority: 3\r\n" ;
$Headers .= "X-Mailer: PHP mailer\r\n" ;


$message = "ชื่อผู้ส่ง : " . $_POST['u_name'] . "<br /><br />";
$message .= "เบอร์โทรศัพท์ผู้ส่ง : " . $_POST['u_tel'] . "<br /><br />"; 
$message .= "ที่อยู่ผู้ส่ง : " . $_POST['u_address'] . "<br /><br />";
$message .= "<br /><br />" . $_POST['u_msg'] ;


ini_set("SMTP","203.150.225.191");
ini_set("sendmail_from", "$umail" );  // [email protected] คือเมล์ของผู้ส่ง สามารถเปลี่ยนแปลงได้ 
ini_set(smtp_port,25);

$mail_ok =  mail($to, $subject, $message, $Headers);
if($mail_ok)
{
?>
		<script language="JavaScript" type="text/JavaScript">
			 alert("Sending mail sucessfull");
		</script>
		<?php
			echo "<meta http-equiv='refresh' content='0; URL=contact.php'>";
}
else
{
?>
		<script language="JavaScript" type="text/JavaScript">
			 alert("Sending mail fail");
		</script>
		<?php
			echo "<meta http-equiv='refresh' content='0; URL=contact.php'>";
}
?>