Email inserita in spam da Gmail

Buona sera, vorrei chiedervi come mai Gmail inserisce in spam le mie email.
Ho fatto un ricerca e ho inserito gli header e validato l'html, ma Gmail continua a mettermi le mail in spam! ecco gli header che uso:

Codice PHP:

---

$headers = 'MIME-Version: 1.0' . "\r\n";
$headers .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";
$headers .= 'From: Sito <info@sito.com>' . "\r\n";
$headers .= "X-Priority: 1 (Higuest) \r\n";
$headers .= "X-MSMail-Priority: High \r\n";
$headers .= "Importance: High\r\n";
$headers .= "Reply-To: NoReply <info@sito.com> \r\n";
$headers .= "Return-Path: NoReply <info@sito.com> \r\n";
$headers .= "Organization: sito.com";
$headers .= "Content-Transfer-Encoding: 7bit \r\n";
$headers .= "X-Mailer: PHP v" . phpversion(). "\r\n";
$headers .= "X-originating-IP: " . $_SERVER["REMOTE_ADDR"] . "\r\n";
$headers .= "Date: ".date('Y-m-d H:i:s', time());


---