in /etc/postfix/
recipient_delimiter = +
recipient_bcc_maps = pcre:/etc/postfix/recipient_bcc

in /etc/postfix/recipient_bcc:
/^(.*)@(.*)$/   catchall+$1=$2@firebird.local

Then run "postmap" on /etc/postfix/recipient_bcc and reload the mail server.

All mail gets archived as it looked when it left the client, with the envelope
recipient (which is the only place BCC recipients actually show up, as BCC is
not a real message header) tacked on the top in the X-Original-To header.