that is exactly what I want to happen. I read that link just yesterday, thank you for posting it. Now, I have a file: /var/root/.forward
and it says: /dev/null (I have not found a root mailbox so far ... where do I create it and with which permissions etc?)

and when I send mail to root from the commandline it gets rewritten to:
root @ mydomain.org and postfix sends it off... and it bounces.
and if I put roadie (my username) into that /var/root/.forward file instead of /dev/null - it gets rewritten to roadie @ mydomain.org - and, since that mail account actually exists on the server, - I do get the mail - but from fetchmail - not what I want - the mail should not leave the machine.

in the article it says to put username@localhost into roots .forward file. good, makes sense but still gets rewritten and sent out. I am reading on how mailqueue and local delivery actually works - but I am slow. Have not found a thread so far that tells how someone set it up in words I understand. What am I missing? ... is it the local_recipient_map in postfix and the postmap command? I get confused with virtual, forward, aliases and all of that.
to me it looks as if roadie and roadie @ domain.org are 2 different identities to postfix - where do I glue them together so that they share the /var/mail/roadie inbox file and local mail arrives direct and not via the mailserver of my hosting company? thank you for giving your time. I am learning heaps btw - great undertaking.