Paypal IPN文件无法向买家发送电子邮件,但对我而言

I have a Paypal IPN file which should send a serial number to a client once I've had payment. All used to work fine. More recently, it still sends a copy to me fine, but hardly ever gets through to the buyer. Here's part of the php code - can anyone see what the problem might be? Cheers, gregston

require '../PHPMailer/PHPMailerAutoload.php';
$mail = new PHPMailer;

$mail->addAddress($payer_email, $first_name." ".$last_name); // doesn't get through
$mail->addAddress('admin@studio-soft.co.uk', "Admin copy");  // is OK
$mail->setFrom('paypal@studio-soft.co.uk', 'StudioSoft');
$mail->addReplyTo('admin@studio-soft.co.uk', 'StudioSoft');

$mail->SMTPAuth = true;
$mail->isSMTP();
$mail->Host = "ssl://host54.servers.prgn.misp.co.uk:465";  // Is this the problem?