Allow automatic BCCing of outgoing email?
When this option is set to Yes, you will be able to configure each virtual server so that email sent by its mail users is BCCd to a separate address. However, this will only work when the Postfix mail server is being used and if the sender_bcc_maps directive is set in its main.cf file.

If Postfix also has the recipient_bcc_maps directive set, you will also have the option to set an address for BCCing incoming messages.