实时服务器上的邮件问题

我使用php邮件功能发送电子邮件。在我的开发服务器中，它工作正常，但在实时服务器中，电子邮件进入垃圾邮件文件夹。

<?php
    $to = "example@gmail.com";
    $subject = "Registration";
    $from = "example1@gmail.com";
    $content = "Test";
    $headers = "From: Test <" . strip_tags($from) . ">'r'n";
    $headers .= "MIME-Version:1.0" ."'r'n";
    $headers .= "Content-type:text/html;charset=iso-8859-1" . "'r'n";
    $headers .= "X-Priority: 3'r'n";
    $headers .= "X-Mailer: PHP". phpversion() ."'r'n";
    $res = mail($to,$subject,$content,$headers);
    $headers = "From: Test <" . strip_tags($to) . ">'r'n";
    $headers .= "MIME-Version:1.0" ."'r'n";
    $headers .= "Content-type:text/html;charset=iso-8859-1" . "'r'n";
    $headers .= "X-Priority: 3'r'n";
    $headers .= "X-Mailer: PHP". phpversion() ."'r'n";
    $res2 = mail($from,$subject,$content,$headers);