the greatest value I see in Zimbra is that it offers a programmatic access to the functionalities, using open standards.
Looking at the source code (zimbra\ZimbraServer\docs), I see a lot of information about the SOAP messages.

Is there a detailed example (e.g. in Java) a SOAP client for Zimbra?
Is there a WSDL?
If not, what SOAPAction should be used?
How is authentication handled? With HTTP headers, using Basic authentication.