The Exchange Server 2007 license comes in two flavors: Standard and Enterprise. Next to the difference in price, there are some technical features that may help you choose between the two editions when choosing a license for your Exchange 2007 Mailbox Servers. The Standard edition of Exchange 2007 allows you to create 5 storage groups, and a total of 5 stores when deploying it as a mailbox server. The Enterprise edition increases that to 50 storage groups, and 50 stores. Two sidenotes here, Microsoft urges you to have only one store per storage group (by default you are limited to 5 stores in a storage group), and even limits you to one store per storage group when configuring any form of continuous replication, and secondly, creating a Recovery Storage Group doesn't count as a storage group. Only the Enterprise edition of Exchange is cluster-aware, so if you need to deploy the Exchange 2007 Mailbox Server role in a windows cluster topology (CCR or SCS), you will need to go for the Enterprise Edition of Exchange Server 2007. And no, there is no database size limit anymore for the Standard Edition of Exchange Server 2007, as there is none for the Enterprise Edition of Exchange 2007!