(I'm still considered a newbie, but I'll take a crack at this anyway.. :-) )
I don't see why it wouldn't be possible. All communications with the server is via SOAP. You can read about the SOAP API in the documentation below. Also, a great resource to figure out how the API works is to use a utility like Firebug to watch the requests be sent to the server from the official webmail client.
http://zimbra.svn.sourceforge.net/vi...raServer/docs/