Exchange 2010 STD or ENT

Server: Standard
CAL: Enterprise

Reasons:
Server: 5 Database stores - 200GB max per store
http://www.infotechguyz.com/exchange2010/exchange2010editions.html

CAL:

The functionality of the client is dependent on the CAL and independent of the Server.

• Integrated Archiving (Ent CAL)


• Multi-mailbox Search & Legal Hold (Ent CAL)


• Advanced Journaling (Ent CAL)

Exchange 2010: Editions and Versions
http://technet.microsoft.com/en-us/library/bb232170.aspx

• No loss of functionality will occur when the Trial Edition expires, so you can maintain lab, demo, training, and other non-production environments beyond 120 days without having to reinstall the Trial Edition of Exchange 2010.


• You can also use a valid product key to move from Standard Edition to Enterprise Edition.


• The RTM version of Exchange 2010 is 14.00.0639.021. The SP1 version of Exchange is 14.01.0218.015.