ok, this is what I mean:
"Inbound" applications to submit tasks, schedule meetings, etc. At present, such bindings are in the form of XML/SOAP network interfaces to the Zimbra Collaboration Server, but we also encourage the development of generic application developer kits (API libraries) for invoking Zimbra services from Java, PHP, Python, C/C++, C#, and so on.
http://www.zimbra.com/community/developer_zone.html
Also, I think I understand that the web client uses the AJAX technology which involves HTML/JavaScript/XML/SOAP. Do you use anything else, JSP for example, to limit the server implementation to be Java only ?