What I'm try to achieve it's pretty simple, and, after googling, its hard to believe I've been unable to find a solution.
What I want is to forward all current messages (already delivered) of an user (michael@mydomain.com) to another external and out of my control account (michael@hisdomain.com). The source server (the ours) is an Ubuntu Server 14.04 with postfix/dovecot.
The only thing that comes to mind is through a bash script, for example, iterating over every plaintext mailbox's message, manipulate their contents to erase headers, and forward them using sendmail, but I think there should be a simple tool or option of a very well-known tool to specify the destination account and the mail filepath to be forwarded.