What backend service should answer in http://jqueryvalidation.org/ remote-method validate?

What backend service should answer in http://jqueryvalidation.org/ remote-method validate?

I am trying to perform a remote-method validate using jQuery validation plugin.

Backend service is properly invoked, but I do not know what should it answer.

I have tried:
$("#myUserForm").validate({
....
rules: {
      user: {
            required: true,
            remote: "api/checkUser"
      }
},
...
});

And at the backend return the following JSON object:
{ "user": "true" }  or   { "user": "false" }
depending on if it should be validated or not. I have checked with FireBug that both answers are generated and that they are identified as JSON objects.

However, validate plugin is always getting this "user" field as not validated, no matter which answer receives from the backend.

I guess I am missing the exact format that should be returned from the backend service or an additional parameter is needed in the definition.

I am using:
jQuery 2.1.3 with http://ajax.aspnetcdn.com/ajax/jquery.validate/1.13.1/jquery.validate.min.js

Thanks,