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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 [...]

In postfix, when you need your e-mails for some specific domains to relay through another server ( your ip is blocked or for some other reason ) you can specify custom transport maps in the postfix config.
You have to setup postfix to use custom transport maps :
in  main.cf enable the transport_maps directive :
transport_maps = [...]