Probably, the reason WSDL does not exist and does not make sense is that Zimbra HTTP requests/responses do not seem to be web-services to begin with.
A web-service will never respond with a JSON response.
Ofcourse there is a SOAP response as well now (achieved by removing the type="
js" from the format element of SOAP request) - but it is not a must. And the SOAP response seems to be added as an after thought (just a guess!).
And, the original request/responses seem to be tuned towards AJAX UI - hence JSON.