Outlook on Exch 2000

We have an existing exchange 5.5 organisation with 2 servers. We are trying to upgrade to exchange 2000. We did all necessary installations and checks like running forest prep, domain prep etc. We successfully installed first exch 2000 server. All works fine except that we cannot access exch 2000 mailboxes. We use outlook 2000 as client. This is still on the test bed..Any ideas...