PHP mail() nada send..

Hvorfor virker min mail() ikke
.......

ini_set(sendmail_path, "/usr/sbin/sendmail -t -f thor@domain.com");

//Contents of an Registration Auto responce email(text format)
$message = "ny besked
";

$headers .= "From: Name<thor@domain.dk>\n";
$headers .= "Reply-To: <thor@domain.dk>\n";
$headers .= "X-Sender: <thor@domain.dk>\n";
$headers .= "X-Mailer: PHP4\n"; //mailer
$headers .= "X-Priority: 3\n"; //1 UrgentMessage, 3 Normal
$headers .= "Return-Path: <thor@domain.dk>\n";
mail("thor@domain.dk","domain.dk - Ny besked",wordwrap(stripslashes($message)),$headers);

..........

Jeg bruger jepponet
Windows NT 5.0 build 2195
PHP Version 4.0.4pl1
sendmail_from: XXX@net-server.dk
sendmail_path: no value
short_open_tag: On
SMTP: MIN IP