not sure but I think it has got something to do with the MX records. I think you can set up the DNS server to forward everything to another server including mials.
Or if you are not familiar with this method you can weld. Write a script that reads the users mails that has got unread flag and send it to the other server. Crone job and everything should work. I dunno... Depending on the number of users every 30mins.