в ответ могу сказать только одно - учимся читать PHP Manual/Help
Код:
$headers = "From: info@example.com \r\n";
$headers.= "Content-Type: text/html; charset=ISO-8859-1 ";
$headers .= "MIME-Version: 1.0 ";
/*notice there aren't any \r\n after the second two header additions. This is what made this version work correctly*/
mail("john@example.com", "An HTML Message", $body, $headers);