local sendmail failure

I have this strange problem can anyone help me to solve it?

I have a dedicated server with linux operating system. I only use PHP
for my scripts.
All mail from all domains sent via website DOESN'T WORK. Meaning that
whatever webform I use to send email it simply desn't send any email. I

tried simple formmail webforms and also the ones that comes with Cpanel

and they simply don't send any emails.


However all email sent via Outlook work without any problems.


What is causing a problem with sending the mail via my websites? The
PHP code to send an email via webform is very simple:
$headers = "Content-type: multipart/mixed\r\n";
$headers .= "Content-transfer-encoding: 8bit\r\n";
$headers .= "From: e...@mydomain.com\r\n";
$headers .= "Reply-to: e...@mydomain.com\r\n\r\n";


$email = "supp...@domain.com";
$message = "test1";


mail($email, "test email", $message, $headers);
?>