致命错误:在 .. 中找不到类“PHPMailer”./contact/mailtest/process.php在第8行

我试图让PHPMailer工作，但它一直给我这个错误：

致命错误：在第 8 行的/home/a4588543/public_html/contact/mailtest/process.php 中找不到类"PHPMailer"。

第 8 行是：

$mail = new PHPMailer();

代码如下：

<?php
$email = $_REQUEST['email'] ;
$message = $_REQUEST['message'] ;
require("class.phpmailer.php");
$mail = new PHPMailer();
$mail->IsSMTP();                                      // set mailer to use SMTP
$mail->Host = "mysmtp-server";  // specify main and backup server
$mail->SMTPAuth = true;     // turn on SMTP authentication
$mail->Username = "me@mydomain.com";  // SMTP username
$mail->Password = "pass"; // SMTP password
$mail->From = "me@mydomain.com";
$mail->FromName = "Online Request";
$mail->AddAddress("receiver@mydomain.com");                  // name is optional

$mail->WordWrap = 50;                                 // set word wrap to 50 characters
$mail->IsHTML(true);                                  // set email format to HTML
$mail->Subject = "Contact Form";
$mail->Body    = $message;
$mail->AltBody = $message;
if(!$mail->Send())
{
   echo "Message could not be sent. <p>";
   echo "Mailer Error: " . $mail->ErrorInfo;
   exit;
}
echo "Message has been sent";
?>