IMPORTANT NOTE: New orders must also purchase a minimum of 5 CALs
Exchange Server 2013 Standard brings a new rich set of technologies features and services to the Exchange Server product line. Its goal is to support people and organizations as their work habits evolve from a communication focus to a collaboration focus. At the same time Exchange Server 2013 helps lower the total cost of ownership whether you deploy Exchange 2013 on-premises or provision your mailboxes in the cloud. The Standard Edition is limited to 5 databases per server.
Microsoft Open Licensing allows the ability to purchase under the same authorization number for two years. Open Business offers volume pricing with only a five minimum license requirement.