PHP mail（）适用于Yahoo，但不适用于Gmail

Question

我的这部分代码会向雅虎发送邮件，但不会向Gmail发送邮件。我还提到了很多相关的常见问题解答＆＃39;关于PHP mail()的文档和文档，但那些对我来说没有用。

if ($_SERVER["REQUEST_METHOD"] == "POST")
  {
    if(!empty($_POST['submit']))
      {
        $to = $_POST['email'];
        $subject = 'Annual Report';
        $message = $_POST['template'];
        $headers  = "MIME-Version: 1.0" . "\r\n";
        $headers .= "Content-type: text/html; charset=iso-8859-1" . "\r\n";
        $headers .= "From: from@example.com" . "\r\n";
        $headers .= "Reply-To: replyto@example.com" . "\r\n";
        $headers .= "X-Mailer: PHP/" . phpversion();
        $headers .= "X-Priority: 1" . "\r\n"; 
        $retval = mail($to,$subject,$message,$headers);
        if( $retval == true ) {
          echo "Message sent successfully...";
          }else {
            echo "Message could not be sent...";
          }
        }
     else
     {
       echo "empty";
     }   
}