Sender BCC Maps allow you to specify one or more senders along with a bcc_recipient address.
When an incoming or outgoing message matches sender_bcc_maps, the message is BCCd to the specified address

A few examples

• BCC all mail sent from a fax or copier to a Fax/Copier archive
• Bob wants all mail he sends from his 3 company addresses copied to his AOL account
• Audit mail from a specific address

Create the sender_bcc file

Here is how I would create the file using pico.
note: when done editing with pico, hit ctrl-x to exit and y to save

sudo pico /Library/Server/Mail/Config/postfix/sender_bcc

Then you can start entering the following:

# sender-address [space(s)]  bcc-address
sender@example.com    archive@example.com
anothersender@example.com    archive@example.com

Alternatively you can use a plain text editor of your choice.

Run postmap anytime you edit the sender_bcc file

sudo /Applications/Server.app/Contents/ServerRoot/usr/sbin/postmap /Library/Server/Mail/Config/postfix/sender_bcc

Tell postfix where to find your new sender_bcc file then reload postfix

sudo /Applications/Server.app/Contents/ServerRoot/usr/sbin/postconf -e sender_bcc_maps=hash:/Library/Server/Mail/Config/postfix/sender_bcc
sudo /Applications/Server.app/Contents/ServerRoot/usr/sbin/postfix reload

To disable sender_bcc_maps

sudo /Applications/Server.app/Contents/ServerRoot/usr/sbin/postconf -e sender_bcc_maps=
sudo /Applications/Server.app/Contents/ServerRoot/usr/sbin/postfix reload