邮件系统需要安装那种认证证书，如何配置那？

<?php
$config->mail->turnon         = true;   // trun on email feature or not.
$config->mail->fromAddress    = 'annjiang@cop.com';      // The from address.
$config->mail->fromName       = 'annjiang';      // The from name.
$config->mail->mta            = 'smtp'; // phpmail|sendmail|smtp|gmail

/* SMTP settings. */
if($config->mail->mta == 'smtp')
{
    $config->mail->smtp->debug    = 2;          // Debug level, 0,1,2.
    $config->mail->smtp->auth     = true;       // Need auth or not.
    $config->mail->smtp->host     = 'smtp.cop.com';         // The smtp server host address.
    $config->mail->smtp->port     = '587';         // The smtp server host port.
    $config->mail->smtp->secure   = 'tls';         // The type to encode datas, 'ssl' or 'tls' allowed
    $config->mail->smtp->username = 'annjiang@cop.com';         // The smtp user, may be a full email adress.
    $config->mail->smtp->password = '123456ANNJIANG';         // The smtp user's password.
}
/* Gmail setting. */
elseif($config->mail->mta == 'gmail')
{
    $config->mail->gmail->debug      = 0;       // Debug level, 0,1,2.
    $config->mail->gmail->username   = "";      // GMAIL username
    $config->mail->gmail->password   = "";      // GMAIL password
}

配置文件如上，客户端如果想接收发送邮件需要安装认证证书，这种加密形式如何配置那？

折腾了很多天，都没好用。

显示的log

SMTP -> FROM SERVER:220 smtp. cop.com Microsoft ESMTP MAIL Service ready at Thu, 5 Apr 2012 17:24:10 +0800
SMTP -> FROM SERVER: 250-smtp. cop.com Hello [nn.n.nn.nnn] 250-SIZE 10485760 250-PIPELINING 250-DSN 250-ENHANCEDSTATUSCODES 250-STARTTLS 250-AUTH GSSAPI NTLM 250-8BITMIME 250-BINARYMIME 250 CHUNKING
SMTP -> FROM SERVER:220 2.0.0 SMTP server ready