Differences Between Microsoft Exchange Server 2003 Standard Edition and Microsoft Exchange Server 2003 Enterprise Edition.

Exchange 2003 Enterprise Edition supports up to four storage groups, with each storage group having up to five databases.
Exchange 2003 Standard Edition supports only one storage group and that storage group can only contain two databases.

Exchange 2003 Enterprise Edition database size is limited only by hardware (with a maximum size of 16 terabytes).
Exchange 2003 Standard Edition individual databases are limited to a maximum size of 16 gigabytes.

Exchange 2003 Enterprise Edition can be clustered on the Microsoft Cluster Server cluster service.
Exchange 2003 Standard Edition clustering on Microsoft Cluster Server cluster service is not supported.

In Exchange 2003 Enterprise Edition the X.400 connector is included.
In Exchange 2003 Standard Edition the X.400 connector is not included.