Mail Function Not Work

the php mail function not work on my server!

when I run this function not show any error too.

for example I run this code:

<?php
echo "ok";
$to = 'myemail@mydomain.com';
$subject = 'the subject';
$message = 'hello';
$headers = 'From: test@domainname.com' . "
" .
'Reply-To:test@test@domainname.com' . "
" .
'X-Mailer: PHP/' . phpversion();

if(mail($to, $subject, $message, $headers)){

echo "sent";

}else{

echo "notsent";
}
?>