I like to migrate an exchange server to zimbra
but we need to have both servers working during migration process and moving the users one by one

what kind of setup i need to have in zimbra to have users in same domain but some in one server and some in old one server.
I think i need to have the zimbra as first server to collect the smtp connections on port 25
and if the user doesn't exist in zimbra server i need to forward the mail to the exchange server on port 25 too ?

any help on this..