One thing to note: calling an external server from zimlet JavaScript is not allowed. This is a cross-domain scripting security limitation imposed by browsers. You can't call from a browser using JavaScript to a server other than the originating server.
However, you can use the Proxy Servlet to get around this security limitation.
1) Call AjxRpc and the URL is the proxy servlet
2) Pass a param for target that is the url + request of the server you want to proxy to.
3) The proxy servlet will take your request, proxy to the external server and send you back the response.
More info on configuring, testing and using the Proxy Servlet can be found here:
ZCS 6.0:Zimlet Developers Guide:Proxy Servlet Setup - Zimbra :: Wiki