You can hack a mail being sent using the BCC command to find other BCC recipients. If you understand the way mail server's work you would find that whoever told you that does not know what they are talking about.

You can find the TO and CC recipients easily enough. BCC recipients are added to a message unless the mail client add them itself for some strange reason.

I know with our smtp product ,SMTP Wizard SSL v2, we send mail using the BCC property and it strips the header from the message when sending them out.

I can explain exactly how this works, if you need.