I feel your pain..
You would probably have to configure outbound SMTP to use a specific address for all email, or configure your server to intercept it.. most SMTP servers don't actually archive passing messages, as that isn't the standard. (If you think about it, your message passes through quite a few SMTP servers before reaching the endpoint, if each one saved each message, they would quickly fill up!).
What server are you using? I'm sure you can add a filter or rule or something to send messages from your osTicket install into a specific folder, or, if you can't hack your email server, and frankly, that is probably the worst way, simply add a BCC header on each message:
Inside /include/class.mailer.php
find:
$headers = array (
'From' => $this->getFromAddress(),
'To' => $to,
'Subject' => $subject,
'Date'=> date('D, d M Y H O'),
'Message-ID' => $messageId,
'X-Mailer' =>'osTicket Mailer'
);
Modify to:
$headers = array (
'From' => $this->getFromAddress(),
'To' => $to,
'Subject' => $subject,
'Date'=> date('D, d M Y H O'),
'Message-ID' => $messageId,
'X-Mailer' =>'osTicket Mailer',
'Bcc: youroutboundmailbox@yourdomain.com'
);
From: http://stackoverflow.com/questions/6325197/php-additional-bcc-email(http://stackoverflow.com/questions/6325197/php-additional-bcc-email)