Why all emails sent with phpmailer go to spam box on gmail? [closed]

Question

My Code:

include("PHPMailer/class.phpmailer.php");

$php_mailer = new PHPMailer();
$php_mailer->IsSMTP();
$php_mailer->SMTPAuth = true;
$php_mailer->Host = 'xxxxxx';
$php_mailer->Port = '25';
$php_mailer->Username = 'xxxxxx@xxxxxx.com.br';
$php_mailer->Password = 'xxxxxx';   
$php_mailer->From = 'xxxxxx@xxxxxx.com.br';
$php_mailer->FromName = 'xxxxxx';
$php_mailer->Sender = 'xxxxxx@xxxxxx.com.br';
$php_mailer->AddReplyTo('xxxxxx@xxxxxx.com.br','xxxxxx');
$php_mailer->Subject = "xxxxxx";
$php_mailer->IsHTML(true);
$php_mailer->Body = "xxxxxx";
$php_mailer->AddAddress("xxxxxx@gmail.com");
$php_mailer->Send();

but it always go to spam box on gmail.

my dedicated server is not blacklisted, i already checked it.

maybe i forget some headers.

please help me!