Web service written in C# for http://www.nameapi.org/ "Disposable Email Address Detector"

I'm trying to create custom webservice client for "Disposable Email Address Detector" for my ASP.NET web application written in C#. I added Service Reference to my project and successfully generated proxy class and whole configuration using http://ift.tt/1FcSUcF

Below is the code which I use to query web service:

SoapDisposableEmailAddressDetectorClient service_client = new SoapDisposableEmailAddressDetectorClient();
soapContext context = new soapContext();
context.apiKey = "myapikey";
soapDisposableEmailAddressDetectorResult result = service_client.isDisposable(context, "test@testmail.pl");
Debug.Print("soapDisposableEmailAddressDetectorResult.disposable: {0}", result.disposable);

Unfortunatelly webservice throws HTTP error 400 BAD request.

I decided to test web service using SoapUI, and I also used Fiddler to check what is send to NameAPI webservice from my app.

Below SOAP message generated by SoapUI:

<soapenv:Envelope xmlns:soapenv="http://ift.tt/sVJIaE" xmlns:dis="http://ift.tt/1eA8QQK">
   <soapenv:Header/>
   <soapenv:Body>
      <dis:isDisposable>
         <context>
             <apiKey>*****</apiKey>
         </context>
         <emailAddress>test@testmail.pl</emailAddress>
      </dis:isDisposable>
   </soapenv:Body>
</soapenv:Envelope>

And SOAP message generated by my app:

<s:Envelope xmlns:s="http://ift.tt/sVJIaE">
  <s:Body xmlns:xsi="http://ift.tt/ra1lAU" xmlns:xsd="http://ift.tt/tphNwY">
    <isDisposable xmlns="http://ift.tt/1eA8QQK">
      <context xmlns="">
        <apiKey>*****</apiKey>
      </context>
      <emailAddress xmlns="">test@testmail.pl</emailAddress>
    </isDisposable>
  </s:Body>
</s:Envelope>

Below implementation of isDisposable method:

public MyNamespace.NameAPI.soapDisposableEmailAddressDetectorResult isDisposable(MyNamespace.NameAPI.soapContext context, string emailAddress) {
    MyNamespace.NameAPI.isDisposableRequest inValue = new MyNamespace.NameAPI.isDisposableRequest();
    inValue.context = context;
    inValue.emailAddress = emailAddress;
    MyNamespace.NameAPI.isDisposableResponse retVal = ((MyNamespace.NameAPI.SoapDisposableEmailAddressDetector)(this)).isDisposable(inValue);
    return retVal.@return;
}

The difference is obvious, but how to fix my app to generate proper SOAP message?