Postfix has the option to bcc all sent or recieved emails for a specific domain.
You can bcc all recieved mails for a specific by inserting the following in to /etc/postfix/main.cf :
recipient_bcc_map = pcre:/etc/postfix/bcc_recipient
then edit the file /etc/postfix/bcc_recipient and add your domain like this :
/(.*)@(example.com)/  backup@example.com
then at the console type postmap /etc/postfix/bcc_recipient to load the bcc [...]