php form mail suddenly stopped to function

[adder]

Hello,
My php form senders just collapsed a couple of days ago. It is a simple script with an HTML form and this code to process the form. If I try to send a test to my colleague sitting in the same room, it goes through, if someone from outside the office is trying to use it, we do not receive anything. I don't have a clue, what has happened.
Any suggestions would be much appreciated!

<?php
error_reporting(E_ALL);
ini_set('display_errors', 1);
$to = "[e-mail]";
$name = $_POST['name'];
$acct = $_POST['acct'];
$street1 = $_POST['street1'];
$phone = $_POST['phone'];
$street2 = $_POST['street2'];
$fax = $_POST['fax'];
$town = $_POST['town'];
$email = $_POST['email'];
$ount = $_POST['ount'];
$person = $_POST['person'];
$zipper = $_POST['zipper'];
$ordnu = $_POST['ordnu'];
$q1 = $_POST['q1'];
$q2 = $_POST['q2'];
$q3 = $_POST['q3'];
$q4 = $_POST['q4'];
$voice = $_POST['voice'];
$subject = "Message From $name";
$message = "Name: $name\nAccount: $acct\nStreet1: $street1\nStreet2: $street2\nTown: $town\nCountry: $ount\nPost Code: $zipper\nPhone: $phone\nFax: $fax\nE-mail: $email\nOrder Number: $ordnu\nPerson: $person\nInvoice: $voice\nTut Pack: $q1\nTut Split: $q2\nEmperor Pack: $q3\nEmperor Split: $q4";
mail($to, $subject, $message, "From: $name <$email>\nX-Mailer: PHP/" . phpversion());
header( "location: redir.html" );
?>

[coopster]

Hi adder, a very belated welcome to WebmasterWorld.

Are you saying that the script works fine if you email to somebody within your office, but if you try to use the very same script on the very same server with an "outside" email address it is failing? You may have to offer some more details on the process, not the code, that looks simple enough, but the steps being taken ...