如何使用PHPMailer发送主题

Question

我使用PHPMailer类。我写了一封这样的测试邮件：

$mail = new PHPMailer();
$mail->SMTPDebug  = 2;   
$mail->Username   = "username"; 
$mail->Password   = "aaa";  
$mail->SetFrom('mail', 'name');
$mail->AddReplyTo("mail","name");
$mail->Subject    = "PHPMailer Test Subject via smtp, basic with authentication";
$mail->MsgHTML('BODYYY OF MESSAGE');
$address = "address";
$mail->AddAddress($address, "name");
$mail->addCC("mail", "name");
$mail->AddAttachment("a.jpg");      // attachment
$mail->Send();

一切都很好，但它没有附加到标题，我看到它仅在被称为DKIM_add函数时附加的代码。如何在没有DKIM_add功能的情况下发送主题？