Вместо этого
$FirstName = trim ($FirstName);
$LastName = trim ($LastName);
$Email = trim ($Email);
$Comments = trim ($Comments);
лучше исправить на
if(!empty($_POST["FirstName"])) { $FirstName = trim ($_POST["FirstName"]); } else { $FirstName = ""; };
if(!empty($_POST["LastName"])) { $LastName = trim ($_POST["LastName"]); } else { $LastName = ""; };
if(!empty($_POST["Email"])) { $Email = trim ($_POST["Email"]); } else { $Email = ""; };
if(!empty($_POST["Comments"])) { $Comments = trim ($_POST["Comments"]); } else { $Comments = ""; };