What's the difference between the standard and enterprise versions of Exchange Server 2003 and Exchange 2000 Server?

The standard and enterprise versions have been with us since the introduction of Exchange Server 5.5. The biggest difference between the two versions is that the standard editions of Exchange are limited to one public Store and one private Store, each of which is limited to 16GB in size. You can't cluster standard edition servers, and Exchange 2000 Standard Edition servers can't function as front-end servers (a restriction that Microsoft removed for Exchange 2003). Exchange 2000 Enterprise Server and Exchange 5.5, Enterprise Edition (Exchange 5.5/E) also include the X.400 connector.