if you want to have bcc of all messages passes through postfix, then you may add line
always_bcc = email@example.com
into your /etc/postfix/main.cf file. This options sends bcc of all messages i.e. incoming as well as outgoing, to firstname.lastname@example.org
Similarly, if you want to tweak the setting, you may use sender_bcc_maps and recipient_bcc_maps feature.
For example: if you want bcc of all outgoing message from a domain1.com, you may create a file with any name in /etc/postfix with something like this:
add following line to /etc/postfix/main.cf
sender_bcc_maps = hash:/etc/postfix/filename
The same could be done with recipient_bcc_maps.
You may find details at http://www.postfix.org/postconf.5.html
Hope this helps, tell me in any further clarification/guidance in the matter required.