If both servers use the same distribution and system version, then replacing /etc/passwd and /etc/shadow on the new server with the old files will give you the same UID/GID numbers. The documentation of your mailserver should explain where and how data is stored, so simply copy these as well. As for the /home directory - copy that one completely. If your home directory is on it's own hard disk, then simply add the old hard disk to the new system.
A lot of possibilities - be creative.
Last edited by Tinkster; 10-30-2010 at 03:35 PM.