mail( ) function problem

Hi everybody,
The following is coding, but the mail is not send. Plz help me..

<?
$from = $_POST['email'];
$contents .= "Name : ".$_POST['name']."\r\n";
$contents .= "Email : ".$_POST['email']."\r\n";
$contents .= "CompanyName: ".$_POST['cname']."\r\n";
$contents .= "Phone Number : ".$_POST['number']."\r\n";
$contents .= "Comments : ".$_POST['comments']."\r\n";

$to = "kumar@spiceware.in";
$from_header = "From: ".$from;
$subject = " Feedbacks from Clients";
if(!empty($contents))
{
mail($to, $subject, $contents, $from_header);
//redirect back to url visitor came from
header("Location: thankyou.html");
}
else
{
print("Error, no comments were submitted!");
print("");
}
?>

with regards,
Krishna