this method emails everyone at once, so the 'to' header will show "name1, name2, name3" and everyone who gets the email will see everyone else's address.
to send each user an individual email, move the email section of the code into the while function, like so...
$query= "SELECT Email FROM Users";
$result= mysql_query($query);
while ($row= mysql_fetch_array($result)){
$send_to = $send_to. $row[Email ].", ";
// format the string for the mail() function
$length = strlen($send_to);
$send_to = substr($send_to, 0, ($length)-2);
mail($send_to, ....);
}