发送电子邮件PHP时出错

我在网上找到了一些php代码，并对其进行了一些修改，使其适用于我。这样做的目的是从我的域中的电子邮件向收件人发送我想要的消息。它对我的一些电子邮件起到了作用，但当我将电子邮件发送到我拥有的电子邮件列表时，我在收件箱中收到了一个错误，并显示了以下文本

Title - "Undelivered Mail Returned to Sender"
Message - "
This is the mail system at host gateway06.websitewelcome.com.
I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
                   The mail system
<wyunnluqxas12@aol.com>: host mailin-04.mx.aol.com[64.12.88.131] said: 550
    5.1.1 <wyunnluqxas12@aol.com>: Recipient address rejected: aol.com (in
    reply to RCPT TO command)"

我用来向收件人发送电子邮件的代码如下所示：

<?php
include("Mail.php"); 
$recipients = $_GET['email'];
$headers["From"]    = ""; 
$headers["To"]      = $recipients; 
$headers["Subject"] = ""; 
$body = ''; 
$params["host"] = ""; 
$params["port"] = "25"; 
$params["auth"] = true; 
$params["username"] = ""; 
$params["password"] = ""; 
// Create the mail object using the Mail::factory method 
$mail_object =& Mail::factory("smtp", $params); 
$mail_object->send($recipients, $headers, $body); 
?>

我做错了什么？为什么它对一些收件人有效（我试过两个人使用@gmail.com电子邮件），而对其他人无效？