How to log/archive full outgoing mails including BCC info?

[Yang Zhang]

How do you get Postfix to keep a log of all outgoing mails, in their
complete form (all headers + payload) as received from clients? The
closest param I've found so far are always_bcc & friends, but these
lose some information (at least the BCC field). Thanks in advance for
any hints.
--
Yang Zhang
http://yz.mit.edu/