Exchange 5.5 on Server 2003

We run a Lotus Notes house but I have one holdout user (a Doctor) with an Exchange 5.5 server running on a Windows 2000 Server. We are migrating the network from WINNT4.0 Domain to Active Directory Domain. The server is not being upgraded to Server 2003. Is Exchange 5.5 going to work still or do I need to be upgrading to a newer version of Exchange?