Zimbra sells annual licenses for the Zimbra Collaboration Suite (ZCS), and support for systems administrators in the support of ZCS. With different features, there are different editions of ZCS, ranging from free (the entirely open source version), to the Network Professional Edition and its options such as Zimbra Mobile (Windows Mobile and Palm), and Zimbra Archiving and Discovery (HIPPA/SOX compliance).

Zimbra charges a standard rate of $28/mailbox/year for the Network Professional Edition, in minimum increments of 25 users ($875/year minimum). If you allow your paid, commercial Zimbra license to expire, it reverts to the open-source edition.   Note that this is the licensing cost only, it does not include, for example, hosting, hardware, operating system, systems administration, and help desk support, it's only the license to run the software.

Zimbra sells licenses directly, and through a network of Zimbra value-added-resellers (VARs) such as 01.com.  Zimbra takes pains to ensure that pricing is the same through either channel.  The VARs typically provide the services to help organizations deploy Zimbra themselves, services such as dedicated hosting, hardware specification, operating system management, migration, integration, help desk services and training. 

Zimbra also has relationships with various hosting providers such as 01.com, who deliver Zimbra as Software as a Service (SaaS), a turn-key solution for organizations to rent the use of Zimbra on a per mailbox, per month basis, securely across the internet.