Just tar them on old server and copy it over to the new server and extract.
Same method goes for both mailboxes and postfix configuration.
Something like this:
Code:
tar -cf - -C /path/to/user/mailboxes | ssh anotherhost tar -xpf - -C /path/to/user/mailboxes
However, like previous poster pointed out, re-creating users on another system is a bit different matter.
How to do that depends what kind of users do you have? System users or virtual (stored in somekind of database, e.g. mysql)?