Postfix V2.1 supports a "sender_bcc_maps" function which is a lookup table of sender addresses. If the sender address matches, the right hand side of the lookup table will be added as a BCC to the e-mail. So effectively, you could add each user you wanted to log mail for, in the left hand side of the table. In the right hand side of the table you would list the admin e-mail address you want to send the BCC'ed mail to, so for example:
In /etc/postfix/main.cf add:
sender_bcc_maps = hash:/etc/postfix/sender_bcc
Then edit or create /etc/postfix/sender_bcc in the following format:
and run "postmap /etc/postfix/sender_bcc" and "posfix reload".
Basically any mail sent through postfix, with an sender address of email@example.com
would also be BCC'ed to firstname.lastname@example.org
Just FYI, you can do this based on recipient addresses to, with a similar format table using the "recipient_bcc_maps = hash:/etc/postfix/recipient_bcc" in /etc/postfix/main.cf
One thing that I think you should add is that for this to work (at least for me), I had to also run the commands: "postmap /etc/postfix/recipient_bcc" and "postfix reload".