address rewriting

[dino muzic]

how can I do this? 
 
1) inside server => postfix => outside:
from:user1@mydomain.local  to:us...@yahoo.com  => POSTFIX => 
from:us...@mydomain.com to:us...@yahoo.com 
 
2) outside => postfix => inside server
from:us...@yahoo.com to:us...@mydomain.com => POSTFIX => from:us...@yahoo.com  
to:user1@mydomain.local
 
 
thanks
 
dino muzic