Php Mail Function Not Working On Site. Need Apache/php Hel 2

Running this script does not send email’s

<?php
$to = “someone(at)example.com”;
$subject = “Test mail”;
$message = “Hello! This is a simple email message.”;
$from = “someonelse(at)example.com”;
$headers = “From: $from”;
mail($to,$subject,$message,$headers);
echo “Mail Sent.”;
?>

I can send directly from the Sendmail server fine.