Microsoft Exchange Server 2010 External Connector grants access to the Exchange server by an unlimited number of external users such as business partners, suppliers, customers, retirees, and alumni.  With this license you do not need to buy Individual Exchange Server CALs.  This EC license is purchased for every copy of Exchange Server 2010 that can be accessed by the external user.  An example of an external user is a person who is not an employee or similar personnel of the company or its affiliates.  It is licensed per server.

Exchange Server 2010 can help you achieve better business outcomes while controlling the costs of deployment, administration, and compliance. Exchange delivers the widest range of deployment options, integrated information leakage protection, and advanced compliance capabilities, that combine to form the best messaging and collaboration solution available.