ขอถามเกี่ยวกับ PHPMailer หน่อยครับ SMTP Error: Could not authenticate. Mailer Error: SMTP Error: Could not authenticate.

<?php
require("class.phpmailer.php");
$mail = new PHPMailer();

$mail->CharSet = "utf-8"; 
$mail->IsSMTP();
$mail->SMTPDebug = 0;
$mail->SMTPAuth = false; //$mail->SMTPAuth = true;
$mail->Host = "mail.co.th"; // SMTP server
$mail->Port = 25;
$mail->Username = ""; 
$mail->Password = ""; 

$mail->SetFrom("[email protected]", "Name1");  
$mail->AddReplyTo("[email protected]", "Name1");  
$mail->Subject = "ทดสอบͺ PHPMailer "; 
$body = "ทดสอบ SMTP Server ทดสอบ PHPMailer."; 

$mail->MsgHTML($body);

$mail->AddAddress("[email protected]", "Name2");
//$mail->AddAddress("[email protected]", "recipient2"); 


if(!$mail->Send()) {
    echo "Mailer Error: " . $mail->ErrorInfo;
} else {
    echo "Message sent!";
}
?>